Controled burn colored fire and safety
I've done some research about it, and have found that the salts of some metals can be used for the intended purpose (for instance, copper sulphate burns green).


You're right! Various melts salts are known to colourize a flame. In the lab and/or for demonstration purposes, this is typically achieved by holding a magnesium oxide stick with a bit of the salt attached into the flame of a Bunsen Burner. There's a wikipedia page on Flame tests that shows some possible effects.


  
    I use kerosene as my fuel[&hellip;]
  


Here's a problem. The salts you could use soluble in water, but insoluble in a solvent as unpolar as kerosene! Consequently, you might want to give a flammable, but more polar solvent a try.

Is it conceivable to change the flammable solvent used in your performances? I reckon that you might achieve the desired effect using ethanol or methanol. With respect to the toxicity of both solvents, I'd definitely try ethanol first!