I agree that if you find that reddish stuff in fired black powder cases, it's most likely burned sulfur. Raw sulfur in nature comes in yellow, red, and even black. The darker colors come from exposure to heat or burning.
Smokeless (nitrocelluose) powder does not have any sulfur in it. what I have found is case cleaner or tumbling media ("rouge"). Rouge is a very fine polishing compound (used in cosmetics and industrial polishing) imbedded in an occasional case. It won't hurt anything, other than taking up space that powder would occupy.
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