Understanding colour models in Affinity applications

[momsoft]

I have been very happy designing my documents using the RGB colour format and the default sRGB colour profile.  I can use the RGB values in websites and other applications (such as Office) and they work. If I need to print the documents, I generate a PDF using CMYK and a suitable color profile, and Affinity takes care of the conversion for me.

But now I need to create a visual identity manual and need to refer to the colors both in RGB and CMYK. What I have discovered is that the CMYK values provided by Affinity are completely different than those used by other applications.

So what is the deal with RGB to CMYK conversions in Affinity applications? I am sure there is an explanation for the differences, but I just can't understand what is happening.

[Mark Oehlschlager]

If you're designing in the sRGB color space, and then allow the Rendering Intent settings in Preferences to convert your color to CMYK numbers in a CMYK color space such as U.S. Web Coated (SWOP) v2, you're relying upon the color engines built into the Affinity Suite to propose a suitable/accurate translation. I presume that Affinity software translation will be quite satisfactory, but the CMYK numbers may differ slightly from another application, one that perhaps is also using a different Rendering Intent (i.e., Perceptual, Relative Colorimetric, Saturation, Absolute Colorimetric).

Having said that, if you're preparing a visual identity manual, when it comes to specifying CMYK formulas for brand colors, I recommend that you refer to printed swatch books from printing ink companies such as Pantone. You want to be sure that the appearance of the printed inks are accurate. You can only assess the accuracy of a printed CMYK color from an actual printed sample. Evaluate the printed swatches under daylight or D50 lighting conditions, select the CMYK colors that best match the sRGB colors you're proposing, and then use the CMYK formula from the swatch book for those colors. (You can also refer to the electronic Pantone CMYK swatch collections included with the Affinity software to quickly add specific CMYK swatches to your document palette.)  

One other thing to note: if you're choosing color in sRGB first, and then seeking a CMYK equivalent, you should know that all of the RGB color spaces (sRGB, Adobe RGB, etc.) have a much larger color gamut than the CMYK color space. That means that there are many RGB colors that are brighter and more saturated than can be reproduced in the printed CMYK color space. For critical brand colors, make sure that those colors can be accurately reproduced in both the RGB and CMYK color spaces. 

[Print Monkey]

Are you using the same CMYK output profile when comparing the CMYK values? If the RGB working spaces are the same, and the CMYK output spaces are the same, and the rendering intents are the same, gamma curves the same, etc, the CMYK output values should be the same.

People usually use a wider gamut working profile than sRGB if they're going to output CMYK.

If you're on a mac you can use the gamut viewer to compare profile gamuts, and if on windows you can use http://www.gamutvision.com/docs/tour_gamutvision.html 

If you compare sRGB to SWOP you'll notice sRGB doesn't include the whole SWOP gamut. If you compare say AdobeRGB to SWOP then you'll see some parts "chopped off". Those will get converted into the edge gamut color when using colorimetric rendering intent.

[lacerto]

I would recommend using a properly set up document with RGB and CMYK color profile pair that is appropriate for the purpose of giving color references. One problem is that Affinity apps limit their display color rendering according to selected document color mode and space so the rendering is not as accurate as could be wished. E.g., if you use Adobe RGB color space, you would get wide enough gamut to fully cover typical CMYK color spaces (sRGB color profile would cut off e.g. some cyan tones). On the other hand, sRGB would most often be the most practical in giving color references for digital usage. If your purpose is to create colors that can be produced both in sRGB (most commonly used digital color space and covered by most modern computers and devices) and typical printing conditions (lightly coated stock), I would work in RGB color mode using sRGB profile and then use Soft proof Adjustment layer with appropriate CMYK profile as a top toggle layer (or clipped into single objects or object groups, if necessary) to get an approximation of how defined sRGB colors would look in CMYK target, or multiple targets (it is not nearly as good as switching to CMYK color mode with appropriate target, especially if you have photos, but acceptable when using for solid color swatches).

If you then use the Color panel while having the lock turned on (it is oddly still more difficult to tell the difference in version 2 apps, at least on Windows and using light UI), you can switch between the RGB and CMYK color models and display CMYK equivalents for sRGB definitions (using the underlying CMYK color profile; note that this can be set as per document if you change color mode to CMYK and specify the profile, accept the change and then switch back to RGB). This would match the CMYK values produced when exported to CMYK PDFs and using the document color profile without changing the color definition itself (just make sure that you have that lock turned on!).

What is important is that the color profiles are always clearly stated both for RGB and CMYK references as otherwise they do not mean anything. Printed swatches with CMYK definitions (e.g. PANTONE CMYK Coated and Uncoated) are kinds of poor substitutes for properly established color managed work environment (meaning that you would typically get a closer CMYK match by letting the color management convert a color to target color space chosen for specific media, e.g., different for glossy, matte, uncoated and newspaper stock). They can be useful if actual printing conditions are close to ones used when producing the printed swatches, and if there is any uncertainty as for accuracy of computer color management.

CMYK equivalents that you get from diverse online services are often mathematical abstractions that should be avoided at all cost.

Specifying colors using L*a*b color model would allow using profile-independent color values, so a good addition (as well as Hex definitions for sRGB).

If you intend to also include special inks in your color descriptions (e.g. PANTONE inks), I would recommend getting printed swatches to have a realistic view of how they look printed and then give the official L*a*b definitions as a reference for simulating the inks in process color spaces. If you need to get closest PANTONE ink for a process color, I think that you cannot use Affinity apps for that purpose and would need to rely on official PANTONE conversion tools (using e.g. PANTONE Connect or PANTONE Color Manager).