That doesn’t make sense to me, probably because it’s wrong, but I can’t be sure it’s wrong as it doesn’t make sense to me 
Use this photo to test channel mixer settings on:
http://rawtherapee.com/shared/test_images/colortarget/Pentax%20K-5%20daylight.dng
Use the colorwheel:
-
Apply the (Neutral) profile, zoom right into the blue patch.
What do you expect will happen if we tell RawTherapee to “add some of the values from the blue channel into the red channel”? Looking at the color wheel, I expect the blue color to shift in the direction of red, so I expect blue to change to purple. -
Using the channel mixer set the blue component of the red color (blue dot in red circle) to 50.
What happened?
Blue changed to purple as expected. But simply increasing the brightness of the red channel (because we added some of the values from the blue channel into the red channel) means that all colors will shift to red. Did that happen?
Yes, the whole image has a reddish color cast. Look at the bottom row of what used to be grey patches - they be pink. -
To remove the color cast, you must subtract from the red component of the red channel the same amount you added to the other components in the red channel.
We set R(B)=+50, so we must subtract 50 from R(R).
Et voila, neutral colors restored, and blue is still shifted towards red, making it purple.
Ditto for the rest.