Not sure whether precedent works better as an ability word or a keyword.

I could see it being a keyword but with reminder text that varies with the card it's on, like Amplify (Daru Stinger). The reminder text would say "this card's mana cost is ".

Those seem to be out of fashion these days though. Transmute (Perplex) or Cascade (Bloodbraid Elf) could have used that kind of wording but didn't.

It could even be a new mana symbol like phyrexian mana, though it's probably not high-profile enough to justify it.

Precedent (As long as you control a permanent that shares a color with this spell, you may spend mana as though it were mana of any color to pay this spell's costs.)

Although that formulation would hamstring a lot of attempts to make it a sane effect on multicolor cards, I don't know if you want to explore that kind of space anyway.

The idea is for this to appear on multicolored cards like Act of Jealously and Noble Follower. It's meant to be a color-fixing mechanic. It could also appear in a set that had a lot of colorless-mana producing lands

"(As long as you control permanents of each of this spell's colours...)", perhaps?

I considered "As long as each of this spell's colors are represented among permanents you control..." So that a single permanent could serve as precedent for a spell.