Huh. That's pretty wacky. So I name green, say, and this makes my Kalonian Tusker cost , Titanic Ultimatum cost , Maelstrom Archangel cost . Looks pretty good, especially for . But I guess it's not quite as good as Chromatic Lantern, so maybe it's okay.

The idea is a colorless, more limited prismatic omen. I didn't consider the impact on multicolor but since it doesn't do it better than omen or (as you note) lantern, I'm fine with it.

I keep reading this as "flirtation stone" :)

Presumably the original intent was more like, "you may spend mana of any colour as though it was mana of that colour", so permanently fixing you in that colour, but not necessarily multicolour spells. But yeah, if Chromatic Lantern is (somehow) ok, this is probably ok too.

I remember I used to play Pulse of Llanowar, back in the day... but yes, Prismatic Omen and Chromatic Lantern (and Joiner Adept before them) show that spending a card doing nothing but fixing your colours is allowed to work rather efficiently these days.