Good morning and thanks for your answers.
@ChasingShadows The tone curves adjust the three channels simultaneously, as explained in RawPedia. I indeed want to adjust the histograms individually. Is there a way to accomplish that? According to RawPedia the RGB curves should operate on the individual channels. But as I had mentioned, if I use the RGB curves, the changes in R also affect e.g. the G channel, and I do not understand why this is so. It is not possible for me to set the shadow and highlight points for an individual channel correctly using this tool.
@patdavid: Yes, of course this will introduce colour shifts. But this is what I am after, to correct a colour shift in the original. In the link you gave this is explained for GIMP, but using the same tool in RT poses the problems mentioned above. It works fine in GIMP, but this is only with 8Bits/channel.