Hello, for printing I need to use colours without any black in CYMK. Is there a method to prevent to use color with black in it ? (it seems it’s possible in PSP). Thanks for your answers and eventual tips.

Are you sure you really need to completely remove the black ink from the CMYK separation? This is normally only needed with special printing processes (e.g., some packaging printers or the color layer of cartoons). In modern times, most offset printers prefer to go the other direction, i.e. they use a pretty high amount of K to reduce total area coverage, save ink, minimize color shifts etc.

That said, I, too, believe that a high amount of K can often be detrimental to the overall color appearance, especially in skin tones, light blue skies, pastel colors and so on. In fact, most designers and photographers would actually prefer to have more CMY and less K to prevent greying and muddy gradients. Unfortunately, though, in most cases, they have no say.

You are correct in that in other software packages there's an option to achieve that. In Photoshop this is called "Custom CMYK". This is an outdated feature, however, and should not be used anymore, as it's a workflow that bypasses modern color colormanagement. Today, all color conversion engines are based on ICC profiles, which have the parameters "baked in".

That means that, in a modern workflow, to achieve what you want, you would need a custom ICC profile that uses Minimal GCR or even No GCR. (GCR stands for Grey Component Replacement: This algorithm is responsible for the appearance of K, when separating images into CMYK.) To date, I have not found any vendor that offers such a profile - let alone, a freely available one. I have created such a profile myself a few years ago, when I needed it. Could look it up, if you're interested.

Sorry, I couldn't be of more help.

Oh, wow, thanks for that tidbit.

marcuschristopher wrote:Are you sure you really need to completely remove the black ink from the CMYK separation? This is normally only needed with special printing processes (e.g., some packaging printers or the color layer of cartoons). In modern times, most offset printers prefer to go the other direction, i.e. they use a pretty high amount of K to reduce total area coverage, save ink, minimize color shifts etc.

Thanks for the answer. Yes, I’m sure about the Black. I’m a comic artist in France and, as you said, editors/printers ask for less than 15% Black. But some ask for no Black at all. In Photoshop, here is the advanced fusion layer setting I have to check this (see capture https://ibb.co/mthqk7Z )

I may try your profile to see if I can use it. Thanks.

lazero wrote:Thanks for the answer. Yes, I’m sure about the Black. I’m a comic artist in France and, as you said, editors/printers ask for less than 15% Black. But some ask for no Black at all. In Photoshop, here is the advanced fusion layer setting I have to check this (see capture https://ibb.co/mthqk7Z )

Hmm, I'm not sure, in what context this setting is helpful to you. But this is not the same as separating the image into CMY, while keeping the K channel empty. What this does is merely to disable the black channel of the current layer, so that it no longer appears in the resulting blend. Depending on the colors you are using, this will result in a totally different appearance, as the neutrals and near-neutrals will now be missing and most of the lightness information (for which the black channel was responsible) will be removed.

Now, I'm not sure, what workflow you are using:

• If your image starts out as RGB (which is the correct workflow with photo-like images that have a lot of smooth gradients) and would simply like to perform a CMYK separation for printing, then I suspect that the setting your are describing is actually not what you want or need! Instead, you would need to perform a custom CMYK separation that results in all RGB colors being separated into the CMY channels only. In this case, the CMY channels would have to compensate for the black channel staying empty, i.e. they would become a bit darker. This is what a custom ICC profile is for. And in modern workflows, there really is no other (advisable) way to achieve that.
• If, on the other hand, you start painting and drawing in CMYK space (which is useful, if your image contains mostly flatly colored areas without any gradients or transitions), you wouldn't need to do anything, as the image is already in CMYK, anyway. You just need to take care to only paint with colors that consist of Cyan, Magenta and Yellow only, i.e. no Black ink.
• If - and this is the worst case - you already have a CMYK image, but would like to simply get rid of the K channel, things become a bit more difficult. You can't simply delete the K channel, as that would modify the colors, of course. In this case, you could either convert to RGB and then back to CMYK (with a custom ICC profile) or go directly from CMYK to CMYK using a so-called Device Link profile. However, I know of no freely available Device Link profile that can do this, as these profiles are normally tailor-made by the printing studios to fit their machines and workflow.

Note: in the last case I believe one doesn't need to convert to RGB and then to CMYK, in Krita going to Image -> Convert Image Color Space and setting up CMYK, then the custom color profile without K or with minimal K will result in converted image, without the losses one might get when going to RGB first (losses because of math and not fitting inside the RGB profile (depending on the profile) etc).

For a dirty cheap trick, I believe I already answered you somewhere - not sure where, on reddit possibly? - but you should be able to use Filter Layer on top of the stack with Filter -> Adjust -> Color Adjustment Curves, with K curve reduced to 0. Alternatively you can uncheck channel K in Channels docker - it can be actually better for painting, since it's probably faster than a filter layer - but I don't think you can export with that, so for export you'll need a filter layer anyway.

Thanks for the suggestions. I work for the moment directly (flat colors) in CMYK picking no black inside colors. I noticed that the "Color to alpha" setting can remove black but, as you said, colors can be dramatically changed coming from RGB.

tymond wrote:Note: in the last case I believe one doesn't need to convert to RGB and then to CMYK, in Krita going to Image -> Convert Image Color Space and setting up CMYK, then the custom color profile without K or with minimal K will result in converted image, without the losses one might get when going to RGB first (losses because of math and not fitting inside the RGB profile (depending on the profile) etc).

True, that might be a good way. I suspect, in this case Krita will internally perform the cross-separation by first converting to L*a*b* and then back to CMYK, so there will also be small inaccuracies involved. But they will be mostly negligible.

tymond wrote:For a dirty cheap trick, I believe I already answered you somewhere - not sure where, on reddit possibly? - but you should be able to use Filter Layer on top of the stack with Filter -> Adjust -> Color Adjustment Curves, with K curve reduced to 0.

Good idea, but this, too, will result in different colors, as the black channel would simply be emptied, without the other channels compensating for the change.

tymond wrote:Alternatively you can uncheck channel K in Channels docker - it can be actually better for painting, since it's probably faster than a filter layer - but I don't think you can export with that, so for export you'll need a filter layer anyway.

True, you can't just export without a black channel, as the resulting file would be corrupt. But you could simply fill the black channel with white in the end. This, too, would result in different colors, however...

Best (and easiest) would be to not use any black ink, while working.

lazero wrote:Thanks for the suggestions. I work for the moment directly (flat colors) in CMYK picking no black inside colors. I noticed that the "Color to alpha" setting can remove black but, as you said, colors can be dramatically changed coming from RGB.

Hmm, if you already do that, I'm actually not quite sure, what the problem is. If you work in CMYK space and pick colors with CMY components only, the K channel will be empty in the end. No need to remove anything. Or am I misunderstanding something?

As to the colors changing, when going from RGB to CMYK: Yes, that is to be expected. No way around that. Not even with custom profiles. That's just the way it is: The gamut of most CMYK spaces is not as big as the gamut of even the smallest RGB spaces.

As the subject says : I want to be sure not to add by mistake a color with black in it. But I suppose you are right. It’s better to work directly without any black and check the colors to be sure I did not do wrong.

lazero wrote:As the subject says : I want to be sure not to add by mistake a color with black in it. But I suppose you are right. It’s better to work directly without any black and check the colors to be sure I did not do wrong.

Ah, now I understand.

I read your first post as meaning that you have an RGB image and would like to convert into CMYK, while keeping the K channel empty. If I (now) understood you correctly, you are already working in CMYK, but would like to make sure not to use any K by accident...

Hmm, I guess, I'm the wrong one to answer that, as I'm better with color management in general than with Krita. But I guess you could simply add a Color Adjument Curves filter layer above your drawing, with the K curve reduced to 0 (as suggested by tymond) and then draw on paint layers below this filter layer. This would have roughly the same effect as the Photoshop setting you mentioned.

marcuschristopher wrote:
Hmm, I guess, I'm the wrong one to answer that, as I'm better without color management in general than with Krita. But I guess you could simply add a Color Adjument Curves filter layer above your drawing, with the K curve reduced to 0 (as suggested by tymond) and then draw on paint layers below this filter layer. This would have roughly the same effect as the Photoshop setting you mentioned.

Quite interesting. I will have a try.

> a Color Adjument Curves filter layer above your drawing, with the K curve reduced to 0

Oh, that's a good tip.
I think I'll try that on a brush with the Filter brush engine. Just to get a tool to do quick local cleaning on highlights and saturated parts.