If you must play with fire like that, with chlorate/sulfur compositions, first at least wash the sulfur with a saturated bicarbonate or carbonate solution (bearing in mind of course, that the sodium ion tends to overwhelm flame colors with its own spectra in the visual range, so perhaps potassium carbonate would be a better choice), and add a little carbonate into the final mix, to mop up residual acid traces from the sulfur, like one would with for instance, nitrocellulose, nitroglycerine, etc. where the classic conc. H2SO4/HNO3 mixed nitrating agent has been used.
