VC paper is sensitive in two bands, to yellow light for low contrast, and blue light for high contrast. So the question is really how much yellow and blue the filter passes, respectively. My own sets (I have two) are of the old type. How would you describe the colour of grade 4/5 on your new set? More yellow than grade 3.5? That would be strange. If it were somehow more blue, you'd have consider that your eyes and the paper do not have the same spectral response, so it is possible that those filters just have a narrower band pass around the blue frequency. But in all honesty, unless you look at the filter spectral transmission and compare it to the paper's spectral sensitivity, you would be guessing, like I am now.
Originally Posted by Michael R 1974