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I took a few artwork pictures in sunlight. I used Rawtherapee's pipette to find a white balance I'm happy with, but the red are overbright.

How can I desaturate reds only ?

I had some success decreasing contrast, but obviously it affects the whole picture.

Here is an example. Left is the original picture, red is too bright. Right is the corrected picture, here obtained with contrast set to -20 (with Gimp) : the red is correct, but not the other colors.

Overbright red

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enter image description here

Left is with HSV equalizer's S-value decreased, right is original. Only a very quick demonstration, so I did not bother to fine-tune it, so please forgive the reddish edges of the blossom, especially in the background.

Probably the easiest way is to use RawTherapee's HSV ("Hue/Saturation/Value") Equalizer:

  • Enable it
  • Create a curve for the S-parameter
  • On the right side of the curve, there is a tool-button that looks like a curve with a cross. Click it, use Ctrl and left-click and drag in your picture the color that you want to (de)saturate
  • Repeat the click-and-drag until you are satisfied.

If you want the red to be darker/brighter, you need to change the V-parameter. If you want to change the hue of the red (e.g. making it more orange or magenta), then use the H-parameter.

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RawTherapee has an HSV tool. HSV stands for 'Hue-Saturation-Value' where Value means luminance or brightness. Other image processing applications call the same type of tool 'HSL (Hue-Saturation-Luminance)' or 'HSB (Hue-Saturation-Brightness)'.

These tools are divided into eight or so color bands: red, orange, yellow, green, aqua, blue, purple, and magenta or similar. They allow one to shift the hue, increase or decrease saturation, and increase or decrease luminance for each color range independently of the other color bands. Rather than shifting all colors by the same amount using color temperature and white balance controls, you can emphasize and/or de-emphasize certain colors without shifting all of the colors. I've always thought of it as kind of a "graphic equalizer" (to borrow a term used in audio processing) for color.

This answer to What is the effect where some objects are a single bright color but the rest is black and white? shows several extreme examples of using an HSV/HSL/HSB tool to reduce saturation to monochrome for all but one or two colors.

This answer to How to edit photos shot in fluorescent light has some detailed examples of how an HSL tool was used to adjust for the color casts to a wide variety of skin tones shot under less-than-full-spectrum lighting.

This answer to Lots of noise in my hockey pictures. What am I doing wrong? shows how an HSL tool was used to remove color casts from different types of less-than-full-spectrum lighting in different areas of the same scene.

These questions show other "looks" or corrections for less-than-ideal lighting that can be achieved using an HSV/HSL/HSB tool:
How are these washed-out, but bright, colors achieved?
How do I get this foggy/cold/blueish look in my photos?
How to deal with shots at night with different kinds of light from different sources?
How can I remove the blueish color tone from some pixels?
How is this sort-of duotone achieved?

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A possible solution in Gimp:

  • In the Channels list, right-click on the Green or Blue channels, and Channel to selection
  • Select>Invert
  • Duplicate the layer
  • On the top layer: Color>Desaturate>Desaturate (about any mode will do)
  • Adjust opacity of the layer to taste

enter image description here

In the general case you would use the channel mixer (Colors>Components>Channel mixer) to create a B&W image where the white is the color you want to work on and then use it as a selection (click on any of the RGB channels and channel to selection as above). This method (which works a bit like luminosity masks) creates a progressive selection and makes changes look very natural. In your image the main color is almost pure red, so you can use a selection on one of the opposite colors as a shortcut.

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