I don't think I'll have it ready in time for the burn, but I wanted to try working with flame colorants for The Widowmaker. Here are some experiments with solid and liquid copper sulfate. I did some experiments with boric acid as well, but those didn't turn out as well. The key I discovered was to make a saturated solution, then use a very shallow vessle to hold it with a very small hole in the bottom; in this case, I just inverted a 1/2" end cap which had a 1/16" hole in it on top of a pipe, and held it in place with rescue tape.
The results were nice, but the gas blew most of it out. Spraying it directly into the flame might be workable, but the timing would have to be just right, and I wonder if I can get enough volume. I don't have a spray bottle sitting around that I want to ruin with copper sulfate just now, so it will wait a bit until I get one from The Jerk Store.