My friend Moe thinks maybe it could be chalks/pastels. If that interests you I may have more archived info on them I can share if you ask. I have used artist's pastels/chalks before and they are not particularly easy to work with and need a PC compatible spray fixative like PYM II (which, at best, produces only a semi gloss, not a matte). Moe suggests one could try PYM II as a spray fixative followed by a painted or dabbed on with a semi-moist sponge) matte varnish sealer. I have not yet found a truly matte varnish, as even the Rustoleum Varathane© satin (matte) isn't really matte, just less shiny. To get a matte finish I sometimes use the satin and just rough it up a little with very fine sandpaper or very fine steel wool after it's dry.
My opinion is that it could pastels/chalks or some sort of a PC compatible colored matte wash or semi transparent colored finish painted on over the PC like an alcohol ink or tinted/colored TLS (translucent liquid Sculpey).....both of which I think I can identify in the picture gallery of her Etsy studio Use arrows on the photos to navigate.
I found this article which may or may not provide some clues.
Hope your search proves fruitful.
Anita in AZ