I think I clearly stated that you expose through the base. Tartrazine would be an option for a light-restraining dye, but does it wash out or stay in the matrix? Tartrazine is actually suggested by Friedman as one of the textile dyes to use in the image transfer stage; so it would need to clear out of the matrix completely or would cause some problems I think. A yellow pigment might be an other option, and this could stay in the matrix indefinitely, as long as it didn't affect the dyes in some way.
For starters though, it's not necessary to complicate things with a light-restraining dye. First it will be necessary to see what can be done with a plain matrix, and only fix problems as they arise.
I've heard about this hue shift with Kodak's cyan dye and their mordant. I need to learn more about making receiving paper, but I can assure you I won't be using thorium! It will be interesting to see what affect a mordant might have on the dye-color, but again, we're talking about an incredibly sophisticated and complicated product by Kodak. The hobbyist need not concern himself with these matters, at the present.