RGB and CMYK - what to expect when printing

Question

I've designed some artwork which I originally planned to only print myself (on my home laser printer), and so I worked in RGB. I've since decided to do a print run in CMYK (outsourcing to a 4-colour offset printer). I'll be working in CMYK going forward, but this is going to be the awkward transition project from my old process.

I've been calibrating my RGB files to print as I want on my laser printer, and have them at a point where I'm very happy with the colours as printed. I am aiming to roughly match these printed colours, not the colours I see on screen (which is to say, I'm already prepared for the limitations of intensity that come from reflective colours versus emitted colours).

I also do not need the CMYK print run to exactly colour match the files printed from RGB (and I know this is not possible). I am simply trying to get a similar 'balance' as far as overall hues and contrast.

I've noticed that when I directly convert the RGB files to CYMK, they appear much blacker on my screen (in addition to the dulling of blues, greens and purples I was expecting). I'm concerned that if I try to print the files without any adjustments, the resulting prints will be extremely dark and dull.

What I am curious about is: if I get the CMYK files as they appear on my screen to closely match the RGB files as they appear on my screen, will the CMYK prints match the RGB prints to a similar degree?

As an example, here is one image I am trying to prepare:

Would it be reasonable to expect that the far right image (when printed on a 4-colour offset printer), should turn out reasonably similar overall to the far left image (as printed on a standard home laser printer)? Or is there no correlation?

I will be getting a hard proof of my CMYK prints, but obviously the closer I can get in the first attempt the better.

Answer 1

CMYK isn't just CMYK

There is no mathematical/logical way to convert from RGB to CMYK. There is no such thing as ordinary/standard/generic/straight CMYK. A CMYK image is just a collection of raster percentages. Instructions for the printing device - not objective colors. It will look differently depending on which exact inks the printer uses, which paper you print on, the device itself, how it's calibrated etc.

This is why color profiles were invented. They are made by agreeing on a set of standard settings for a given device and then printing physical swatches which are measured to create conversion tables.

When you convert an image from RGB to CMYK those conversion tables are used to make sure that your image will look as similar as possible to what you see on your (ideally expensive color calibrated) screen when printed on a device which follows a certain standard.

So it's important that you find out which profile your print house recommends for the paper type you are printing on and use that. Using a wrong CMYK profile won't do any good.

Don't create your artwork in CMYK

Unless you know exactly what you are doing it is not really recommended to work in CMYK. For most users converting to CMYK should be seen as a final step to prepare your files for a specific printing device. One time you might use a print shop which uses one CMYK profile and the next time you need to convert to another. No need to limit your artwork to one specific profile from the beginning.

Furthermore, working directly in CMYK enables you to make colors which would never occur if you just converted from RGB to CMYK and the result might look dramatically different from what you see on screen.

It looks like you are pretty skilled at painting in RGB. If you work in CMYK it would be a lot harder to get nice gradients and you have fewer blend modes and effects to choose from.

CMYK in GIMP

I have tried to convert your image to "ISO Coated v2" (a very widely used profile for coated paper) using rgb2cmyk.org and when I open the result in Photoshop it looks almost similar to the original. The darkened image you are seeing could very well be wrongly displayed. See the answer of @BillyKerr which describes GIMP's color mangement better than I can.

Delivering the image for print

Have you asked the print shop how they want you to deliver the files? Where I work we don't accept image files as final files. To me an image is just an asset not a print document. It could be scaled, rotated, cropped in many different ways. Someone (if not you) would have to take your image, place it in a layout program, make design decisions, add bleed if needed and finally create a PDF for print.

I normally wouldn't convert images to CMYK manually. I convert when I export to PDF from my layout application (InDesign, but you could maybe use the free alternative Scribus as suggested in a comment). This way I can easily make alternate version for different paper types in the last minute without the need to have multiple versions of all my images. They are all just RGB, but while I work in InDesign I can preview them as if they were converted to a specific CMYK profile. This is called soft proofing and it's one of the reasons why Adobe's applications is still the best/only choice for professional print production.)

Since CMYK conversion is a problem with your current software, you might want to ask the print shop if you can just deliver your files in RGB and let them do the CMYK conversion. With professional software it's only a few clicks.

Answer 2

Since you mentioned GIMP. Here's how to preview what a CMYK print of your work will look like, using soft proofing. I assume you already know GIMP has no CMYK mode support, but we shouldn't need that for just previewing it.

1. Go to the International Color Consortium website, and download one of the CMYK colour profiles. In this example I chose the one named SWOP2006_Coated3v2.icc - Obviously you need to choose the profile for the print service you are using. Contact your printer to get this if necessary.

2. Install the ICC profile on your system. In Windows 10, right click and choose install. Sorry don't know how to do this on Mac or Linux, but I'm sure you can look it up.

3. Navigate to C:\Windows\System32\spool\drivers\color and you should find the ICC colour profile in there. Sorry, don't know the location in Mac or Linux, but I'm sure you can look it up

4. Open your illustration in GIMP

5. Click Edit > Preferences, and select the color management tab

6. If you have a monitor profile select it, otherwise leave it set to None. Shouldn't matter too much although you'll get more accurate results if you have a profile for your monitor, and calibrate your monitor. Set rendering intent to perceptual, and use black point compensation.

7. Set the soft proofing profile to the one you previously installed. You will have to navigate to the location to find it. Again set rendering intent to perceptual, and use black point compensation.

8. Click OK.

9. Now in GIMP from the main menu click View > Colour Management, and check the options as shown below. Also set the Soft Proofing Rendering intent to Perceptual.

Note you can show out of gamut areas by enabling the Mark out of Gamut Colors option. You can use the out of gamut warning preview when creating your artwork, so as to avoid using out of gamut colours in your RGB file.

Finally when you are ready to preview the CMYK print, enable the soft proofing option.

Here I show the an original RGB image, side by side with the Soft Proof, and also with the out of gamut color warning set to bright purple (this shows the out of gamut colours in the RGB file). You will also notice how the brighter blues and purples are somewhat muted in the proof, but this is to be expected.

Click on the image to enlarge

As to actual CMYK conversion, others have addressed that here. But your printer should be able to do it for you. It takes only a few seconds, and obviously they will have to use the same ICC profile for the conversion. If they won't or can't, then find a printer who will or can.

Update: If you want free software to make a CMYK image yourself, then Krita (which is also free and open source) has CMYK support built in.