While working on GoPro footage I noticed some camera-internal image processing with respect to red colors. On a beautiful autumn evening I shot the same scene in the same lighting conditions with the standard Hero3-internal image processing and without, the latter is the so-called Protune mode.

During grading I then noticed that the GoPro seems to "boost"(for lack of a better word) especially reds. GoPro doesn't increases the gamma of the red channel. Instead, it looks as if they selectively increase either the luma or the saturation of a narrow range of colors with the hue=red. With Protune enabled, GoPro skips this boost so I get more neutral colors. However, depending on requirements I would like to emulate this boost in post for some scenes shot in Protune mode.

I've checked all the Kdenlive effects but couldn't find one that does what I described above. I was able to emulate the desired grading effect in a very limited way using the Hue curve: it is a flat diagonal, except for a small red range where I basically increase hue slightly ... this basically slightly shifts orange and red colors.

Did I miss an effect here? Or has someone an idea how to assemble a hue-sensitive luma or saturation effect? That is, I need to specify a hue (better: hue curve) that defines the hue range to be processed, outside the range no changes happen and towards the range limits changes will be much less than towards the middle of the range. The pixels of matching hue should then be changed in luma or saturation. Any idea how to do this?

Put two copies of your video onto two tracks, exactly in sync, and add composite.
Do the desired change (to all colors) on the bottom track.
On top track, use select0r to make the color you want to change transparent.
If you want your selection to be based on hue only, use HCI space and set the I and Chroma deltas to maximum.

Thank you, that's basically what I was looking for. Unfortunately, that didn't give the desired effect I wanted, but that's a different story. So I need to think of other ways in which GoPro handles the red colors. Hmmm...