color nowadays is not directly used by anything. It's effectively just a builtin CSS variable (via currentcolor) resolved at used-value time.
It is used in quite a few places: https://searchfox.org/mozilla-central/search?q=nsStyleColor%3A%3AmColor&case=false®exp=false&path=, though I guess you mean in the style system itself?
So it probably makes more sense conceptually to move it into upper level object if possible, especially given its small (which may be changed at some point if we start supporting wide-gamut color, though).
I agree with that, though it's more work (we'd need to special-case that property, the style system code generation assumes right now that all properties are on style structs). In terms of effort, it's way less work to move it to
nsStyleText. But after I removed the old
scroll-snap implementation I guess this is not blocking anything really, so we could spend a bit more time on it.