[Jamie]

If Arran's idea won't work, you could also try creating the two different colors, then pouring a "swirl" in first. (Think of it as squirting ketchup in the bottom of the mold). Just dribble the first color on the bottom of the mold. Let it set up, then pour in the main color on top of the swirl.

Hope you get it figured out. You are doing something that's been needed for a very long time.