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]

Obsolete.