'Superficially red (and yellow below the surface)" - some yellow phenolic beads (and other types of jewellery such as brooch pins and bracelets) were resin-washed to make them darker red. In these cases it's not dye as such but liquid phenolic resin, as on this bead.

Coloured dyes were definitely used on blonde casein beads, giving that amazing cut through effect when the deep carving shows up the lighter material below the dyed surface.

Carole