I would try it with one cola, but do it immediately after chopping. Just take a freshly chopped cola, and put the stem in a vase or glass or water colored with food coloring. Within a few days, the cola should have sucked up some of the coloring and colored the buds, then you can dry and cure as normal. I'm not sure how much it would take though. I have one ready to chop in a couple of days, and I think my wife has some food coloring somewhere, so maybe I'll try on a bud..