color correction - How can I desaturate reds in post-processing using Rawtherapee (or Gimp)?

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.

Answer

^{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.