Castin' Craft makes their own opaque and transparent pigment. Normally, Michael's or Hobby Lobby have these in their store. Also Wood-N-Whimsies
(http://www.woodnwhimsies.com/) has all the colors listed on their website.
I have had success with Mixol tint. It is a transparent coloring but I can add a little white solid pigment to make it more opaque. One thing with the Mixol, DON'T USE ON Alumilite. It will foam the resin.
I took the photo to the left from US Composites website. I primarily use the pigments they sell made to color polyester resin. (http://www.uscomposites.com/pigments.html) You can mix these pigment to make other colors. Just get out your old color mixing wheel. I started purchasing 1 ounce bottles then moved up to the 4 ounce size. Like mica, a little goes a long way. These are opaque pigments so there would be no need to paint the tubes with these blanks. US Composites also sells 4 transparent pigments that are fluorescent.
I have tried many other things that I didn't think worked at well. Things like Testor model car paint, finger nail polish, acrylic paint and properly others. I even tried powder coating products that do not mix well. My advice is to stick with the colorate designed for resin. Feel free to comment below on other things you have used to color your resin. Below are blanks that I made with solid pigment.
I'm a beginning woodworker playing with simple projects I can complete quickly.