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I have an image in RGB which I would like to silk-screen on a t-shirt. I want to prepare the file so that the result is as close as possible to the original image.

I could simply convert the image into CMYK, but being for a silk-screen print I would like to use as few plates if possible. Perhaps the black one is not needed? Or maybe if I used two or three different plates the result would be sufficiently close for the given particular image?

So my question is:

How do I convert (and preview!) my RGB image into duotone or tri-tone as bunch of spot colours?

Addendum: The question got edited few times so I will try to rephrase it: how to convert RGB image into 2 or 3 spot-colours, so that the end result is as close as possible to original data? The conversion might (or might not!) make use of the colour of the medium (i.e. black or white t-shirt).

  • I will update my answer if you post an example image. – Rafael Jan 13 '17 at 18:45
  • @Rafael Thanks a lot for the answer! However, I don't yet have any image in mind, I was asking about a general way how to approach the problem. – CyanMagentaYellowBlack Jan 13 '17 at 20:57
  • @Rafael Ok, just for demonstration sake, let's take this picture... – CyanMagentaYellowBlack Jan 13 '17 at 21:00
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    The general way to do this is to make the color separations manually. Since this is a specialized skill you will find that quite a few people actually do not know how to do this especially since we rely on automated separations. – joojaa Jan 16 '17 at 6:33
  • @joojaa "to make the color separations manually" - how? – CyanMagentaYellowBlack Jan 16 '17 at 11:30
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The process of separating images for screen-printing takes a whole lot of practice, trial and error, and patience. Being a good separator is almost kind of an art in its own right. In fact, there are statistics that show that over 80% of all screen-printing problems occur before the screens even make it to the press. Excluding screen printing equipment difficulties and preparations, most of the problems occur starting with the separations.

One of the major obstacles is that most graphic designers/separators have zero hands on knowledge of actually registering the screens on the press, pulling squeegees, exposing screens, Etc. In fact, almost every single screen printer I know, blames the separator and the separations for unacceptable screen prints. Communication between the two teams is crucial.

Doing separations can range from simple 1,2,3 simple spot color separations which are completely solid colors with no shading, all the way up to 10 or maybe 12 colors containing all sorts of half-toning for photorealistic screen prints on white or dark garments. Before doing any separations, there will always be information you will need before you start the process. For example: what will the garment sizes be? You will need two different sets of separations if you have sizes ranging from youth small all the way up to adult double XL or triple XL sizes. Garment color also needs to be considered. Will there be only one garment color or all different colors? For example, In most cases if you are printing on a black garment, you may not need to use a black screen. So you go ahead and do your separations without including the black screen and then, uh oh.. At the very last minute, The customer decides to have the design printed on royal blue shirts also… which will need a black screen, after you have already completed the separations and printed the films. Another scenario could be that the design was intended to be printed on white shirts only, then at the last minute the customer wants to print on black shirts also. The problem with that is that 99% of the time you will need a white underlay screen as your first ink color so that the other colors printed won't be affected by the garment color..(If you print yellow ink on a blue shirt, the result would be a dull looking greenish color because in essence, combining yellow and blue will give you green). Another thing to consider is there are some images that when printed on white visually look correct but that same image printed on a black garment does not look correct. Sometimes the fix for this could be as simple as just inverting the image and doing a second set of separations for the black shirts. My point to all of this is that knowing these things will dictate the way you will approach your separations.

You posted a comment that I will try to simplify for you.

My question is this: given RGB raster image and how to separate it into two or three plates? I mean, precisely, how do I do that? I know that " the techniques will have them screened and you will use probably more ink from program to program", that I can assign spot colours if there are only two colours. "will have them screened and you will use probably more inks", yes, this is what I try to avoid. – CyanMagentaYellowBlack yesterday

Using the image you posted as an example:

@Rafael Ok, just for demonstration sake, let's take this picture... – CyanMagentaYellowBlack 2 days ago

Having been in the screen printing industry for 30 years now, I just know by looking at an image, whether it needs to be color corrected for screen printing. The image in that link you supplied was way too dull with low contrast and low saturation for printing on garments. The image on the left is the original and the image on the right is color corrected for screen printing.

I created a set of Photoshop actions for color correcting images for screenprinting with a brief explanation dialogue before each command is executed. Download the action here

enter image description here

If this image was only going on white garments, we could do 4 color process separations. Four color process is the technique of using only four ink colors (cyan, magenta, yellow, black) or "CMYK" Those four colors printed on top of each other and blended together can create hundreds, if not thousands, of secondary and tertiary colors.

Just to make life more difficult I decided let's separate this for black garments. This is known as "simulated process" in the screen-printing industry. In essence, we are "simulating" the 4 color process by using ink colors chosen from the design itself, with nontransparent plastisol inks. The result usually creates more screens than CMYK.

So your question was "given RGB raster image and how to separate it into two or three plates? I mean, precisely, how do I do that?

  1. Now using your color corrected image, we simply start out in Photoshop by going to menu item Select/Color Range

enter image description here

Looking at this specific design, I just intuitively know it's going to take six screens if we are going to use a black screen and a gray screen. So I know I can separate this design into White underlay, black, red, blue, gray, and a highlight white screen. Now that I know the colors I will use for screen printing, I will use the color range selection menu to select each one of those colors and create new spot channels with those color selections.

enter image description here enter image description here enter image description here

  1. Continue the process for each color you want to add for your separations (keep in mind that it may take multiple attempts for each color with the color range tool to be able to select the right amount of each color you want to use) the next image is the final results with all my spot channels added along with the shirt color as a background. Keep in mind we are trying to simulate the original image which uses thousands of colors and blends with only six spot colors.

enter image description here

@CyanMagentaYellowBlack there are a few things to consider with CMYK printing on garments. CMYK a.k.a. four color process screen printing is intended for white garments only because the inks are transparent and if there is any other color other than white underneath the printed inks, the resulting screenprint will be unacceptable. Also consider this… CMYK screen printing relies on the white shirt color to be used as the white part of the image. This is what CMYK a.k.a. four color process screen printing would look like on a black garment…

enter image description here

So to sum it up, there is no RGB printing method like there is a CMYK printing method. RGB is only a, for lack of better term, a color space used in computer graphics. There are no RGB screen printing inks or an RGB printing process in comparison to CMYK. So for this image, CMYK screen printing can be used only if the garment colors will be white only. For any other garment colors, this entire post explains what needs to be done for screen separations

  • Thank you! In the example above, you used six plates. Isn't it easier to convert the RGB image into CMYK and print it with half-tones as it is done in plain old offset print on paper? And you have a valuable experience and knowledge I don't posses: is my only change really just trial-and-error and looking at Photoshop preview how the channels mixed? I guess if there was one you would have told me... – CyanMagentaYellowBlack Jan 16 '17 at 11:29
  • @CyanMagentaYellowBlack I am going to add my answer to your question in your comment in my original answer because it's too much information to reply to in a comment – wch1zpink Jan 16 '17 at 21:10
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You can not preview a file that is not prepared.

A 2 or 3 separated inks probably need more preparation than a full 4 color separation.

For example take a look at this post for making an interesting duotone. Preparing design for duotone printing?

But all depends on the image. RGB do not say anithing. Is it a logo? is it a full color photography?

Yes, you can use the blackness of a black Tshirt, but there is not a direct way to preview, you need to make manual changes.


Flat images

I. One ink

You can simply send an image as a grayscale. Make sure that if you want a solid color you have a solid black. Then ask the printer to use any ink you like.

enter image description here

II. Two colors

This becomes tricky. If you just send your RGB file this will be converted to color separation, a CMY+K file But most likely you do not want that, except for photos or simmilar images.

You will have them screened and you will use probably more inks that necessary.

enter image description here

So you need to separate the colors (depending on the program the method changes)

And you simply assign a spot color. That is what I mean specially prepared.

That is not an RGB green it is a spot green for example "Grass green No. 5" and the orange is "Florida Orange No. 3".

enter image description here

But the techniques varyes from program to program. This kind of images are easily prepared when they are on vectors. But on Bitmaps, besides checking the resolution you need probably mask zones and make a multichannel file.

For photos take a look at the link I provided, but normally you just send a CMYK for color separation.


On dark Tshirts you normally print an aditional color, white as a base. You can also print this white base twice times.


A spot color is not "trial and error" at least for you, is to see samples of the ink aplied to the same material you need to print. So you probably want to see printer's trials and errors to see the results.


An aproach for raster images

1) Make sure the resolution is ok for the project.

2) Select the color you want, for flat colors it is easier but the concept is the same. Make a decision on what will be what color. This is not made by chance, but as a decision.

One option is using color range. Make a selection.

3) On the channel pannel make a new channel, choose "New Spot Channel".

Choose one spot color of the libraries.

Choosing that color is not random either, it is again, a decision, probably made by choosing a color seeing a Pantone booklet, or using the colors of a brand manual.

4) Save this file as PSD.

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    Be aware that screen printing on dark T-shirts almost always requires printing white first wherever there are bright colors -- otherwise the colors look like mud. – user8356 Jan 13 '17 at 18:54
  • What do you mean by "a file that is not prepared"? And "to make manual changes"? Are you saying my only option is to separate the RGB data into, say, two plates, print it and see it? Basically trial and error? – CyanMagentaYellowBlack Jan 13 '17 at 22:31
  • My question is this: given RGB raster image and how to separate it into two or three plates? I mean, precisely, how do I do that? I know that " the techniques will have them screened and you will use probably more ink from program to program", that I can assign spot colours if there are only two colours. "will have them screened and you will use probably more inks", yes, this is what I try to avoid. – CyanMagentaYellowBlack Jan 14 '17 at 8:29
  • To put it differently: the problem is that the inks used in offset printing don't knock-out each other, meaning you can blend the ink when you print two overlapping areas. If you print in CMYK, the ICC profiles usually contain conversion matrices also from and to RGB and CMYK so you can soft-proof. Not so with spot colours, esp. the inks used in screen printing. Hence no soft-proofing, no easy way to convert to different colour space as the profile lacks the matrices. Pantone even guards the conversion formula. So what could one do, besides trial&error and "varyes from program to program"? – CyanMagentaYellowBlack Jan 14 '17 at 8:38
  • " 4) Save this file as PSD." Would be appropriate only if the separations would be printed directly from Photoshop itself. If any text or vector objects need to be added after-the-fact, my suggestion would be adding those elements in illustrator. The proper way to do this would be to save the Photoshop file as a DCS 2.0, then fire up illustrator, create a new document, then go to menu item File/Place and select your DCS (eps) file using the link option. Then go ahead and add your vector elements or text and print your seps directly from illustrator. – wch1zpink Jan 16 '17 at 22:24

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