I have a large roll of the receiver sheet here. It is still made last I looked on the internet. The roll I have is opaque base virtually identical to the Ciba/Ilfochrome support.

The colors like this are often created from either natural pigments or from metallized dyes which are complexes of dyes and metal salts. Either way, some of these can be quite toxic as for example the yellow once used Cadmium Yellow as the coloring pigment. The raw "glop" can still be purchased. The last I handled these was at the Formulary where they had a stock of them for making colored carbon transfers in workshops. They are still sold, according to what I have been told, but by whom I do not know.