Which resin are you planning to use to cast this? For Alumilite Clear, you can put the dye into just Part A before mixing in Part B and get a fairly good idea of the final color...it will lighten slightly when you mix in Part B (since you're doubling the amount of resin).
If you work with Alumilite White, you can do the same, but when it goes off, it will add a lot of white which will significantly lighten whatever color you've mixed.
If you haven't worked with Alumilite previously, make a test batch or two without dye to get a feel for how much you need to stir after mixing A and B together. They're designed to start a bit cloudily and turn clear when properly mixed...but with dye added, you won't see that effect. So, you need to know how long to stir if you go that route.
I use it when figuring out new color combinations, then put the target amount of dye into a cup (one for each target color), mix all of the A & B for a batch together, then pour off what I want into each target cup and just stir in the waiting color. That's giving me better results than carefully measuring out the dye after A & B have been mixed (since time is limited once they're mixed, this helps keep things moving smoothly).
I'll even sometimes put a tiny bit of Part A into the dye cup so the dye can be mixed into solution rather than sticking to the bottom of the cup. You just have to be careful not to throw off the ratio of A to B by using non-trivial amounts of A this way...
Alumilite's "Violet" dye mixed with Alumilite White produces a fairly light purple. One drop goes a long ways with their dyes!
