I have a logo which has been designed using RGB colours. Part of the logo is in grey and the colour use for this is a middle grey, R127, G127, B127.

Several online colour scheme generators convert this to C0, M0, Y0, K50. e.g. https://coolors.co/7f7f7f-7f7f7f-7f7f7f-7f7f7f-7f7f7f

This makes sense to me at it is 50% black ink.

Photoshop however converts to C50, M39, Y39, K21.

Photoshop is configured with:

RGB: sRGB IEC61966-2.1

CMYK: Coated FOGRA39 (ISO 12647-2:2004)

As far as I'm aware those are the defaults - I've not consciously changed them. I'm based in the UK.

What is the reasoning behind each conversion method, and which would be the best to use when sending a CMYK version of the logo to a commercial printer?

  • I find it really really naive to think that the color is linear. So C0, M0, Y0, K50 is really suspect. But yeahthis is why logos are designed int cmyk first then converted to RGB. CMYK has one more degree of freedom so conversions have some leeway, anyway photoshops conversion is visually more accurate although it does not handle k the way you want. – joojaa Jul 4 '19 at 18:53
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    Hey Chris and welcome to the site! I can see that your question is getting a somewhat lukewarm reception. Don't take it personally. It's just way more complicated to explain than you might realize. The answer would have to tell you about the basics of RGB, CMYK and color conversion. I'm still trying to figure out a way to explain it in few words. That said, you probably (I can't be sure without seeing your work) should use the 50% black ink (or another percentage) instead of a CMYK gray because it will always look neutral even if the colors of the print is slightly out of balance. – Wolff Jul 4 '19 at 20:21
  • Hi @Wolff, thanks. Do you have any links to good resources where I can learn more? I understand how RGB and CMYK work, just not the conversion process between them. joojaa hints that the conversion process is not linear, which makes sense and goes some way to explaining why the best value may not be K50 however it doesn't explain why photoshop whats to make grey using CMY and a little K, whereas the other converters just use K - which is what I would intuitively think to be the correct method. Is photoshop trying to be too clever? – Chris Wheeler Jul 5 '19 at 13:37
  • We are running some print tests using photoshop's colour, as well as a few different K values with no other colours to see which we think works best. – Chris Wheeler Jul 5 '19 at 13:39
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    Different implementations of GCR (Grey Component Replacement) or UCR (Under Colour Removal) might explain the differences you found. – Stan Jul 5 '19 at 14:07

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