So, this does circumvent color requirements, like phyrexian mana. Originally it was just a simple mana cost, but tracking what mana was produced by what land seemed like obnoxious bookkkeeping.

Hm, it's an Albino Troll for all colours.

I don't think you need the colour-circumvention. How about:

> Deplete (You may cast this spell for its deplete cost. If you do, lands tapped to produce this mana don't untap during your next untap step.)

Or is that what you were meaning by "obnoxious book[k]keeping"? It is fairly easy to circumvent with things like Orochi Leafcaller or Skyshroud Elf.