A quick search suggests that using shotshells with smokeless and chalk is the safest way to do this:
http://www.thehighro...p/t-342945.html
Sounds like a bad idea trying to do this by altering / mixing in the propellant, nevermind the potential legal issues.
Im also wondering if the contents of a `Party Popper` could be adapted/substituted with powdered (coloured ) chalk?,.......... this perhaps could be fired/timed with a slight delay from the hand/finger/trigger to create a safer effect? = techically he would not be altering the BP propellant or adapting the gun directly?