Gunslinger mage, magic moons

Hi all, I'm looking for a book I once read but google is coming up blank, and AI search engines keep hallucinating books that don't exist.

Book MC is a gunslinger mage, that is part of a protective order, who is cursed with the Taboo magic of one of the several monochrome coloured moons.

The moons rise and set, so different ones are in prominance at different times, affecting the magic of the world.

Beginning story beats was that MC is hiding in and protecting a frontier town, But a FMC comes to town and suddenly they both are on the run from Protective order that MC is part of.