First, SSR isn't SSG, it's SSGSS, nobody knows if SSG can have another color.
Second if SSGSS Zamasu (Black) had pink color only because his ki isn't the same as Goku's that would mean every SSGSS should have a different color, but Goku and Vegeta have the same exact color.
Third they already confirmed it's because Zamasu is a god, not because his ki is different.
Fourth, almost everyone has the same ki color. Rainbow ki is Toei's invention, in Toriyama's colored pages from the manga the ki is always white, only a few exception exists (Freeza's death ball, the few ki barriers to ever appear in the manga, the Genkidama and the sparks from the auras).