I have been using gorilla clear glue to glue in my tubes for one specific reason: I can mix acrylic craft paints (which are water-based) with the Gorilla Glue - water activates the glue). I just take a toothpick full of paint of whatever color I want to use, and I stir it into the glue. I then apply the colored glue onto the tubes and insert them. I twist/rotate the tubes as they are inserted to help spread the glue evenly. I then wipe off any excess glue and let it sit overnight. This has worked flawlessly for me for several years now. I used to use epoxy or thick CA, and I always had issues with blotchy colors inside the blank.