This is a red Composition I found that I think works nicely......
Strontium Nitrate : 62
Potassium Nitrate : 15
Sulfur : 19
Charcoal : 4
No Chlorates / Perchlorates -- No Metals
Is all of that oxidizer necessary? Appreciate that you don't want to taint the red coloration with too much charcoal fuel, but seems this would burn up pretty quickly. And no chlorine donors--more expensive than potassium nitrate (cheap in the US at least) and probably strontium nitrate, too, but PVC or Parlon in substitution for some of your oxidizers might give a nice balance between color enhancement and burn rate, and still have no metals. I've personally don't recollect ever seeing a star formulation that used that much strontium nitrate supplemented with KNO3... Seems fuel-poor and chlorine-negligent. But I'm sure it burns quickly. And red. Strontium is forgiving. On ground burns, do you have a lot of solid residue left over? I'd imagine so.
Appreciate aiming for no perc/chlorates or metals, but is this what we're looking for? Certainly somebody's tried something similar before.
What does it look like coming out of a stargun?
Have you tested this "in flight" or are you just making an educated guess? Not nicking you--just asking for additional details. Did you mix/test powder, formed stars on the ground, formed stars in motion? Stars in a completed device?