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If you had a printed artwork that used only Pantone Reflex Blue U, Rubine Red U, and 802 U (Neon Green) -- some colours are pure, some are blends, some are at 100% density, others are less -- and you were then provided with an RGB scan of this print (saying the unprinted paper happened to have an Lab value of 100 0 0), could you separate the RGB file into the corresponding spot channels (Reflex Blue U, Rubine Red U, and 802 U)?

Would you use the same method if there were 5 spot colours, instead of 3?

From what I can tell, converting to Duotone/Tritone/Quadtone would be unsuitable, because it requires converting to Greyscale first.

  • I don't think there are any easy fix for this, especially when there are halftone blends. Here is a guide on color separation, perhaps it can be of help. – Mikael Carlsson Feb 26 '19 at 16:20
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YES it can be done but it's no easy task.

How difficult it would be depends entirely upon the artwork.

One would use a multi-channel Photoshop document and create as many spot channels as are necessary. Then, on the channel, you simply paint or create black/grey areas for the color. Or, recreate things in Illustrator assigning spot colors.

There's no "automatic" method via a few menu commands to complete such a task. It would involve selections, and painting on channels, or creating objects in Illustrator. You couldn't merely scan, run a few command, and end up with proper separations.

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Hi there Leon and welcome to GDSE. Good question (it's one of my favorite subjects), but unfortunately there's no short answer.

Converting from RGB to CMYK has been totally standardized. Anybody can separate a color image into the four CMYK channels with a click.

This has been made possible by a lot of experimenting with different printing conditions, doing lots of test prints and measuring and systematizing the results.

Different color profiles has been created for different printing standards on different kinds of paper. In Adobe's applications you can even get a pretty precise preview of how your artwork would look in real life.

Pantone printing is not standardized in the same way. There are 1800+ Pantone colors so there are billions of combinations. It's impossible to make color profiles of them all.

It is still possible to separate an image to 3 spot colors, but it will take some effort and there are many different ways of doing it.

If your budget allows it you can buy custom made color profiles:

  1. Pay a print house to make a custom color profile for your set of colors specifically for their printing press and for the chosen kind of paper.
  2. Pay a site like Color Library (only example I know) to create a more general color profile.

This is probably not a real possibility and depending on the artwork it might not give the best results anyway, so here's some suggestions for manual separation in Photoshop (some more obscure than others):

  1. Use adjustment layers (for example Channel Mixer or Black & White) to create a grayscale image for the mask of each spot color channel. There are many different ways to do this and it might also involve a bit of pixel painting.
  2. Convert the image to a Custom CMYK profile with a set of custom Ink Colors and Black Generation set to None. Afterwards you have to use the CMY channel masks to make the correct spot color channels. (This has worked for me on color images, but it doesn't work too well recognizing single ink areas in my experience).
  3. Use adjustment layers (for example Hue/Saturation or Channel Mixer) to manipulate your image until the colors of your image has been mapped to the three RGB channels (in this case r: Rubine Red, g: 802, b: Reflex Blue). Each of the channels should be "clean" without traces of the other colors. You can then use the RGB channel masks for your spot channel masks (and then probably continue editing).
  4. Like above but where CMY is the target colors (in this case c: Reflex Blue, m: Rubine Red, y: 802). After correction convert to CMYK to separate to channels.
  5. Scale your image to 400%. Convert to Indexed Mode, 4 colors (your 3 colors + white). Convert to RGB Mode. With Magic Wand (unchecked Continuous) select each color and create separate channels. Scale down to original size.
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Chapter 1: I give up

I tried one method I commonly use to make interesting duotones using channels (see the former answer on the bottom)

Here is my test image using the colors you asked:

enter image description here

And here are the split plates:

enter image description here

But I can't split some channels clean enough to be usable.

enter image description here

Probably I could twist the hue of the original RGB image so the colors are more aligned with the primary ones... but I doubt it will be usable anyways.

Chapter 2

Probably you can vectorize your image and fill the shapes manually.

Chapter 3

Original answer:

Take a look at some of the posts I made regarding "creative" duotone separations. I explored some methods to "find" a Pantone color.

With the colors you mentioned (A red, a blue, a green) It is suitable to use the same RGB channels as a base. But some tweaking could be necessary, and it would depend on the style of design, and how flat and uniform the tones are and the gradients.

Some of these tweakings could be masking some specific regions, adjusting the contrast, etc.

Take a look at these:

Preparing design for duotone printing?

Printing photographs when job is a 2 spot color job

On the second post, it is clear that one problem is that you could use different Pantone colors because one Pantone can potentially be similar to another one at some xx%. But as you have your base inks well defined, you have a good starting point.

I will try to post some images with "flat style" designs, instead of photos.

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