So you have combinations of red and blue colour values (R,B) that correspond to a some arbitrary single values V according to the colour scale. Perhaps 2D interpolation, will work for you:
Extract all the the (R,B) combinations for each pixel (Rq,Bq)
Vq = interp2(R,B,V,Rq,Bq)
If the green channel is also important, use interp3.