Oh, interesting ability, and even more interesting color choice for it. My natural thought process would have me pondering if it belonged in blue or black, as those are green's enemies. It doesn't really fit in black to me, but it might make sense in blue; I might then change it to "spell or ability" so that it felt like an opposition to both red and green. If I weren't thinking of enemy colors, though, I would probably put this in white, since white has a history of limiting "extra" things like spells cast and cards drawn.
It would never occur to me to put his in red, as red restrictions on things often feel unnatural, as red is the color or freedom. It's also odd for red to hose its ally color like this.

This was so unusual an idea I couldn't quite figure where to put it. I just dumped it in the color that IMO has the biggest design space issues ^_^;;

That's true. It's something important to consider when you find new space.

I'd've thought this isn't so much hosing green as hosing red. This doesn't affect Llanowar Elves, after all, but it utterly shuts down Seething Song. Given the historical precedent of Dark Ritual I could definitely see this in white.

Flavour-wise this feels like telling people to slow down, calm down, don't do things so hastily. Which feels white or blue.

Things that don't fit in one color? That's what multi-colored is for!

Seething song is a spell, not an ability.

Whoa! I completely misread that. Right, this only affects abilities? Weird. Okay. I guess that is mainly green then.

The biggest category of cards commonly seen around our tables that this would hose? The Ravnica bouncelands, Boros Garrison and friends. What is this meant to do to them (or Crystal Quarry), anyway? I assume the player tapping the land for mana chooses one of the colours in question to get mana of?

Heck, it's a rare, with an effect that's not gonna happen much - go the whole hog and hose 'em with "It adds instead" to clear that up.

This also hoses the old wording of Mana Flare but not the new, so you're fine there :)