The Azo emulsion is white because it absorbs very little visible light. A true blue sensitive emulsion is yellow. A green sensitive emulsion is red and etc.
There are internal reflections between layers and with the support, and there are internal reflections. These are compensated for by acutance dyes that reduce internal reflections. The AH layer works for the support.
Ives information is seriously out of date. The CLS layer, the 3rd in a color tripack, is the only truly deep yellow layer and it is there to prevent the passage of blue light to the underlying layers.
I've said it before and I'll say it again.... Watch out for old works that are much before 1945 - 1970. and go to the original if you can because of serious errors in derivative works. For this see the handout that I gave at class Chris.