I don't understand why you worded this way. Why not simply say that you're required to spend different mana?

": ~ gets +1/+1 until end of turn. Spend only mana of different colors to activate this ability."

I kept tripping over the wording. Don't ask me why, but I just couldn't get those two sentences out of me. My brain just goes blank sometimes. That said, I wasn't against the idea of people being able to activate their ability, and having nothing happen if they didn't pay two different colored mana, so I tossed in that. It isn't standard, though, and people don't like to learn things that they don't need to learn, so thank you for the correct wording.

Fun little card.

It is fun. It seems excessively restrictive for not being a huge boost - could it be +2/+2 instead?

Yikes. That'd suddenly be very efficient. The only time I've seen pumping that efficient has been on Shades (Nantuko Shade etc) and on Ursapine. This is a little common; I'd guess it's not intended to be a powerhouse.

Very efficient - but you can only do it at either +2 or +4; +1 and +3 are unavailable to you, can never go +5 or higher?

Doesn't seem too horrible.

Oh, it certainly can go +6, or +8, or +10. In a deck, I can pay to activate this, then do that again, then do that again 3 more times. Same in a deck. That's giving rather more efficient power pumping to several colours than they normally get.

+1/+1 is definitely sufficient. Artifact creatures shouldn't get good P/T pumping; even Steel Hellkite costs to get +1/+0.

Oh; I was reading "Spend only different" to have memory. So you'd need , then , then

Hmm... my intent was similar to Alex's method. You should be able to play this in any two color deck and go big with it. That said, I'm not sure how to not lead people down the wrong path... I can see why, Vitenka, you assumed the different colored mana needed to stay different... I just can't figure out how to stop that thought process. I guess I just need to remain receptive and let drafters play with it, with no imput from me, to see if this is a common problem.

As for the stats... they're morphable. This card has a high likelihood of having a different casting cost and p/t based on the needs of the set, so I'm not too concerned with what's fairest right now.