I got a bottle of gun blueing. I dumped it into a Kallenshaan (sp) laser blank container and that is what it lives in now. Whenever i need to darken tubes, I just plug em with some spud, and stick em in the blueing. A little shakey shakey and they are ready to be removed. I let em dry for a few hours before i use them. Local, Cheap, fast, and dependable. It is full of floaties but they don't bother the process.
Otherwise, various colors and variety of paint if i am in some kinda of hurry to glue, or want a different effect. Right now, my favorite is some spray paint i get from a local mom and pop home improvement shop for $1 a can!