I doubt that it would discolor a cola or root beer soda. It probably WOULD change the color of a lemon-lime, ginger ale, or similar drink. It might also make the soda taste funny, though with lemon Cokes coming out nowadays that might be a good thing.
To make colored granulated sugar at home, put a bit of sugar in a jar with a tight fitting lid, add a couple of drops of food coloring, close the jar, and shake it. I have no idea if this also works for powdered sugar, but I’ve used it to make blue granulated sugar.
I’d try the unsweetend Kool Aid, I’ll bet that it makes the soda fizz nicely. Plus it’s very cheap, so you can experiment with it.
Oh, and when filling the capsules, fill them over a paper towel, or a plate, or even over the sink, because you WILL spill some powder and you want to be able to clean it up easily.