I use liquid dyes made for laquer based paints. Any tint should do. Maybe even black india ink. Use only a tiny bit, the less the better.
Use a vacuum pump for pulling a vacuum on it, just hook some fish tank tubing to the tube, then to vacuum pump. it'll bring the epoxy into the tube. There is not a good way to promise no air voids, it just takes practice. Pull the least vacuum it requires to pull the epoxy through the tube. Keep a small vacuum in the setup until the epoxy sets.
I am talking about the handheld vacuum pumps that you can bleed brakes with on a car.
Mityvac, I believe, is the name of mine.
Dan