They have powdered pigments on the website, you probably want the
titanium white. I just looked and it seems they've redone the website, so
things aren't easy to find. (and most of the pics are gone)
I've got an ArtStuf catalog here somewhere, and I have some of the
powder.. I'll take a look and send you the catalog number, or I can send
you some of the powder if you want.
You can also get the liquid/paste dyes from US Composites. They have
an opaque white liquid dye that can not only make the resin white, it
also makes your translucent dyes opaque. I use that a lot, since I don't
care for the tint of some of the opaque dyes, but I like the color of the
translucent dyes. Add white and it gives you the same color, only opaque.
Let me know..