Peacemaker normally glows orange, when shooting demons, revenants, witches, whatever.
However, we've seen it glow blue twice: once when Wynnona shot Willa, and once when Waverly shot at Rosita.
It's not because of who's shooting it, clearly. It's not because of the character of whom it's shooting at (Rosita might not have been so horrible, but Willa was pretty bad, and in any case it still glowed orange when Wynnona shot a few nice revenants previously). And it doesn't seem to be about shooting non-demonic things, either, because (even if witches are demonic), it glowed blue when shooting Rosita, who's definitely a revenant.
So what's up with the blue glow?