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I would like to convert a lot of RGB images to a CMYK where the black is 20, 20, 20, 100.

By making a custom CMYK and using separation type GCR and 22% UCA I can get pretty close. But I can see on the gray ramp that the cyan is higher than the rest. When I convert I get a CMYK black that is 14, 15, 24, 100. So its not perfect.

Is there a beter way to to this? Or can I change how the gray ramp works?

Thanks!

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  • Does your "black" in RGB is always the same value or does it change?
    – SZCZERZO KŁY
    Jun 10, 2019 at 10:10
  • @SZCZERZOKŁY It changes from image to image. I have a InDesign document where the background is 20, 20, 20, 100 and I will have a lot of images with black background that I want to match.
    – imax
    Jun 10, 2019 at 11:09
  • Then you can do that in Acrobat. There is a color management tool that let you switch colors in certain range into specific one. So for example all between 0,0,0,90 and 15,15,15,100 into 20, 20, 20, 100
    – SZCZERZO KŁY
    Jun 10, 2019 at 12:22
  • I really do not understand why you are trying to do this... Just choose the correct color profile with the TAC you need.
    – Rafael
    Jun 12, 2019 at 8:09
  • Perhaps you could consider changing the black in your InDesign document to match your converted images, and save yourself a lot of hassle.
    – 13ruce
    Jun 12, 2019 at 11:57

2 Answers 2

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Color management

The whole idea with RGB to CMYK conversion is to create a CMYK image which on print will look as similar as possible to the original RGB image.

Since there are different printing devices, different kinds of paper, different inks and different ways to calibrate the equipment etc., the same set of CMYK values will look different on different setups. A CMYK color is not an objective color, it's simply a set of technical instructions for the printing device on which halftone percentages to use.

This is why color profiles were invented. They are mainly based on measurements on actual physical prints, not theoretical assumptions. Each print house decides which standard they are going to follow (for each device, paper type etc.) and calibrate their equipment to match.

So if your screen is calibrated according to a standard (often sRGB or Adobe RGB) and the print house has calibrated their equipment according to a standard, Photoshop is able to show you a (more or less) precise preview showing how your CMYK colors would look on print.

There is no such thing as generic CMYK. The only way Photoshop can show us a preview on screen is because it knows (or assumes) which CMYK color profile is in use.

In other words: If you don't know which CMYK profile your print house use, you can't do a proper RGB to CMYK conversion and you can't trust the preview on your screen!

Your custom rich black isn't neutral

You have chosen a rich black with the values (20, 20, 20, 100). This seems a little random to me (although I know that it's common practice for many). The CMY inks does not have the same properties so mixing them equally will not create a neutral color. Your color will have a slight color tint depending on which standard the print house follows.

See here how an RGB black converted to CMYK (darkest possible neutral color) compares to your custom rich black when proofed with different color profiles:

A CMYK conversion will try to maintain the colors of the original. You are trying to force the darkest part of the images to have a specific non-neutral color, so you are (unintentionally?) trying to use CMYK conversion to change the perceived color. This is normally a thing you try to avoid.

Your custom rich black isn't very black

Uncoated paper usually allows 260-300% total ink and coated paper 300-340%. It differs from color profile to color profile.

The total ink of your custom rich black is only 160%. This can make it easier for the printer if you have large areas, but as the darkest point in an image, it's quite low.

A bright red color in the image might be around CMYK(0, 100, 100, 0) so it has a total ink of 200%. That's more than you allow in the darkest part of the image, so the transitions in the image might end up looking rather strange and some kind of banding might occur.

How it could be done using Custom CMYK

The only way I can see for you to force the darkest colors to CMYK(20, 20, 20, 100) using Custom CMYK is quite hacky. You would have to enter Ink Options > Ink Colors > Custom...:

Then you would have to manually adjust the CMY inks and overprints until you have invented a set of colors where equal amounts of ink gives a neutral color.

The result might look OK on screen, but the preview would be based on a custom set of inks no-one uses in real life. So to get a correct preview you would have to then assign the CMYK color profile provided by your print shop and a noticeable shift of colors would occur.

How I would do

The simplest way of matching the dark areas of a photo to a background in InDesign is in my opinion:

In Photoshop

  • Make sure that all images are RGB and that they all have the same color profile.
  • Make sure that the darkest point of every image is RGB(0, 0, 0).

In InDesign

  • Make sure that Document RGB is the same profile as used in the images.
  • Place RGB images in InDesign.
  • Use an RGB(0, 0, 0) swatch for the dark areas.
  • Convert to the correct CMYK profile on export. Make sure to choose Convert To Destination (Preserve Numbers) to preserve Black text (and other deliberate clean inks).
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  • Thank you! Your idea of how to do it sounds a lot better :)
    – imax
    Jun 10, 2019 at 14:08
  • Is there anyway to reduce the amount of color If I use your method? The problem is that the black gets so much color in it that the printer think its to much. @wollf
    – imax
    Jun 10, 2019 at 14:38
  • Are you using the color profile provided by the printer when you convert from rgb(0, 0, 0) to CMYK? Converting images from RGB to the correct color profile will make the darkest area of the images the same as the rich black you make. So it's not too much colors for the darkest areas of the images, but the printer maybe thinks it's too much color for all the black backgrounds? Too large areas?
    – Wolff
    Jun 10, 2019 at 14:43
  • (You misspelled my name, but it doesn't matter :-). I'll automatically get a notification when you are commenting on my answer.)
    – Wolff
    Jun 10, 2019 at 14:51
  • exactly, its a 200 meter exhibition with black background on all the surfaces, printed on Forex print board. So its a lot of just empty black that they think will become too saturated to look good. But I guess it's their job to find a solution :) @Wolff
    – imax
    Jun 10, 2019 at 15:04
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Following are steps to convert the RGB image into CMYK:

1) Open an image on Photoshop.

2) Go to Edit> Convert to Profile.

3) Select your targeted image.

4) Select the relative colour metric “Specific Black Color”

5) Do not use image> mode > CMYK command to convert your image

6) Conversion controls set through “color Setting” application

Now, your RGB image has changed into CMYK.

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  • I'm sorry, but I don't understand how this lets me set a specific black? Where do I "Select the relative colour metric" to "Specific Black Color"?
    – imax
    Jun 10, 2019 at 11:34
  • I can't find the "Specific Black Color" option either. And I don't understand how you can first "Convert to Profile" and then "Select your targeted image"?
    – Wolff
    Jun 10, 2019 at 14:38

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