Judith, thank you for those suggestions. I'll try to grow some madder and weld next year for pigments. Is madder Rubia tinctorum and weld Reseda luteola?
Did your husband use egg tempera? I've heard that it makes extremely durable works.
Have either of you used spores as pigments? In mushroom identification spore prints help to positively identify species, and most have interesting colors.
.....weld (reseda tinctoria) mignonette family; woad (isatis tinctoria) mustard family; and madder (rubia tinctorum)...