I recently fragged a sinularia (finger leather) and I just cut the pieces off, put the toothpick through them and attached them to little rocks with a rubber band. They've all attached now, and it's been about 10 days.
I don't have a colt, but I understand they're pretty slimy and hard to work with. The sinularia was pretty slimy, too, but it worked.
For best results (and to keep your toothpick from pulling through the coral), bore a hole in the rock you will be mounting it on, a few millimeters deep and as big around as the "stem" of the frag. Try to push the toothpick through the frag about as far up the stem as the hole is deep. Then when you attach it, the stem of the frag will go down into the hole and the toothpick will rest on the rock. This will keep the toothpick from pulling all the way through the frag. (I didn't use this method on my sinularia because the frags were really big, with bases of around 3/4" in diameter. Nevertheless, one of them did pull through and I had to remount it with the toothpick.)
Some may also recommend super glue. I've tried this in the past without success. Even if the frag is securely attached, it can still pull away and go floating.
HTH