The high-contrast layer is blue sensitive, so the correct answer is blue. What magenta does is filter
out the complement low-contrast color (green), so that only the blue gets to the blue component of white light. Some red light gets thru too, but the VC emulsion is blind to this. This is just basic color theory, i.e., additive versus subtractive. But given the fact that primary blue filters are quite dense,
it is generally easier in practice to work with subtractive magenta. I use both methods, and identical
results can be achieved. But if you do additive split printing, you need a blue tricolor like a 47,
not just a light blue tungsten-balancing filter, which will still allow a considerable amt of geen thru.