The Fate Reforged Oracle update bulletin mentioned

> Atalya, Samite Master; Crimson Hellkite; Crypt Rats >
> These three cards all have an activated ability with an X in the activation cost that's restricted to a specific color of mana.

Why has there never been a green X activation or a blue X activation? Put this to rights!

Note: There's not very much difference between an ability like Crypt Rats's and one like Pestilence. Crypt Rats is clearer that you can activate it for 10 even if the Rats only have 1 toughness: you could do that if the ability was just Pestilence's, but newer players might get confused. (Also it works better to give Crypt Rats lifelink this way.)

So ideally, you'd find abilities of the form ": Do something with X" that have a justification for not being ": Do something 1 time". That's the tricky bit here.

FWIW, I remember comments on Demonspine Whip on http://archive.wizards.com/Magic/magazine/Article.aspx?x=mtg/daily/ld/40 saying for repeatable abilities, wizards weren't sure between and . But I agree, there's a benefit to being not so you don't have to spell out "spend only".

Fascinating challenge. I still mentally assume monocolour decks, so I have to remind myself that "spend only G" makes a real different to the strength of the effect, not only flavour :)

I made Form-Carver as the most obvious example I could think of that required X y could also feasibly require only green mana for both developmental and flavor reasons.

­Ooze Collector

­Hana Hana Hydra and Cephalid Mind-Wiper

I made Rhonite Artisan and forgot to link to it here.

That wouldn't be very hard for green. I imagine it would be something along the lines of Green Sun's Zenith or Chord of Calling.

As for blue, I made Power of Thought. The wording is a bit confusing, but I think it makes enough sense, and I think the effect is fairly balanced.