Thats going to be your problem. Most opaque colors, it seems to me are the basic colors black, orange, red, yellow, white, brown, pearl colors, flourescent colors, silver and more.
All though I guess all colors could be transparent with very little color added, like smoke(black).
Havent trid the crayons yet, but I would think they are all opaque?
Not all transparent colors are marked "transparent" from the manufactures.
Hope this helps and dont fell dumb.................oh and the
slick...
Willy
might be a good name for the next will that comes along