Two points to add here:

1. The matrix is exposed from the back! This causes the tanned image to begin forming at the surface of the film support for the purposes of adhesion. The yellow dye (tartrazine) is added to limit light penetration all the way to the surface thus confining the tanned gelatin image to be sharp and close to the support.

2. The best receiving material contains a mordant which attaches to the dye. It can often shift the dye hue by a considerable amount. The mordant used in Kodak's process was a Thorium salt and was slightly radioactive. It caused hue shifts that were slight but could be easily seen. I have seen a mordant shift a magenta dye to a good cyan dye upon attachment, but this is rare.

PE