I have a few questions about this.
1. Is this the proper wording?
2. How powerful is this? It needs constant fuel, and will need two spells a turn unless you have multicolor cards. Should it also trigger on abilities?
3. Should this produce colored mana?

It's funny that Multiverse gives this a colored frame automatically, because it should just be "land" colored.

I assume the intent is, if you cast a green and a white spell this turn and it doesn't count if you spend GW to pay ?

I'm not sure about strength. Token-making abilities and abilities on lands are almost always stronger than they look, but assuming it does require two spells of different colours (or a GW deck), that's probably a fairly difficult hurdle to pass regularly, so it may actually be about right, but I'm really not positive either way. I think it's probably fine for usual decks, so the question is, is there anything it combos with too well, and I don't think so.

I think generic mana is probably right, but "ETB tapped. : or ," would probably also be fine. I'm not sure which is preferable on colour-matters lands.

Looking back, it should probably say "If you cast a green spell and a white spell this turn, [thing]."

Yeah, you should override this to the "Land (colourless)" frame. The land-frame-guessing code is fairly smart, but it'll never be able to cope with all the wacky ideas people come up with ;)

As currently worded I'd expect it would trigger every time you spend and on a spell, even if that spell is a Dimir Signet or a Myr Retriever. So it turns Change of Heart into a Lab Rats++. Not that that's exactly broken.