Almost 20 yrs ago & back in my muzzleloading days, it was not uncommon for my Browning muzzleloader w/ a 70 grn FFFg load to show red flakes.
From what I've read, the old-timers considered this "proof" of a well-balanced load....
I doubt if taggants are added to BP as BP leaves more than enough residue to easily identify it. Also the taggant issue w/ smokeless (AKA "White Powder") is apparently a dead issue as the taggants adversely effected the chemical properties, stability, storage life, etc. of the powder.