I usually mix all of my colorant into part A, then add part B. Alumilite gives you a lot less open time than PR and i want to be sure to get the color well mixed into the resin before I add part B so it's not using up my open time to mix the two parts together, than mix in colorants as well. Now since you mix equal parts of each side you need to add more colorant than you would think to part A as the color will get diluted when you add part B otherwise your resin will be more transparent (if using clear resin and powdered pigments).
I have gotten good effects by adding a small amount of a different color powdered pigment to the mixed resin just before I pour into the mold. I just give a few stirs to incorporate it into the resin but not mix it in well. This can give streaks of color through the blanks but less pronounced than swirls of a different color resin.
When I do 2 color casts I measure the two cups of part A, mix in the colorants then mix in part B to both as quickly as possible and pour. It helps if you can have someone help you so you don't waste too much time mixing and the resin set up in the cups.
And as much as I say you need to mix fast, I mix the two parts together for at least a minute, scraping the bottom and sides to ensure it is fully mixed together. You can see when they're fully mixed if you don't add any colorants but it is hard to see with pearlex or other colorants mixed in.
