Something like 20 years ago PAPER TIGER PRESS released a book profiling a selection of fantasy/sf artists including Jim Burns, Rodney Mathews, and Berni Wrightson as well as Vess. The words which I remember describing the latter's medium were "a shellac-based dye" which, while yielding the bright colors Vess is noted for, didn't lend itself to his subtle blending, so his work is especially wonderful.
Does anyone-else remember this book? Does this make any sense? I am reaching back 20 years, and for all I know "shellac-based dye" defines most if not all colored inks.
I know that he used colored inks for years and probably still does. Shellac based dyes is just a fancy name for colored ink, like Pelikan brand. Inks are waterproof when they dry so you can layer color on top of each other for glazing effects.