Curtis,
I love that blank, but you made it backwards! The red should be on the left side! :bulgy-eyes::laugh:
I've never worked with Alumilite, so I don't know it's consistency. With that said, here's an idea that MIGHT work... (YMMV)
Build a "vertical" mold that will hold your cactus blank on end. Only the end on the top would be open. A silicon mold, or other flexible mold, would be best for ease in removing the finished blank. First, put the cactus blank, sized for a snug fit, into the mold. Then pour the blue resin, followed by the white and red. I would think that by virtue or running down the skeleton of the cactus there would be more than enough resistance to keep the colors from mixing a lot, if at all. You could either premeasure the exact quantity for each color and make that amount, or just make a bunch and eyeball it.
Anywho.... my moment of inspiration has passed. Do with it as you will. :bashful: