I'm trying to write a function that will let me red-shift or blue-shift a bitmap while preserving the overall brightness of the image. Basically, a fully red-shifted bitmap would have the same brightness as the original but be thoroughly red-tinted (i.e. the G and B values would be equal for all pixels). Same for blue-tinting (but with R and G equal). The degree of spectrum shifting needs to vary from 0 to 1.

Thanks in advance.