My understanding is that it's okay as long as there isn't any of the (V) vinyl in the material. I just put into inventory, several 4x8x1/4" sheets of acrylic in an assortment of colors so I can just cut letters or graphics in any of those color choices. Black, white, brown, ivory, dark blue and green, yellow.
i also found 6" wide rolls of double sided mounting tape. I put that on the bottom side of the acrylic, cut my letters out, then all you do is peel the paper backing off and stick in place. Makes for really fast dimensional letter application.
for custom colors, I use base white then shoot with Matthews.