The vapor is water vapour dragged by the CO2. So it's white just like a cloud. I'm unable to think what you can put in the water to chage the color of the smoke. Everytime you put some colorant in the water, the smoke is always white. You can lit with coloured light, or you could put some volatile coloured substance to go with the water and the CO2, but I can't think in one right now. Any ideas?