I don't think it's a given that the apple is going to be a green or yellow type. It looks like an immature fruit to me and easily one of the multitude of red varieties. Or, it could be one of the green or yellow varieties, it's too soon to tell at this point.
Give it at least another month. You'll be able to tell when it's getting ripe. It will probably turn color, but even if it doesn't, it will soften a bit and you'll be able to smell that it's getting ripe. One thing to keep in mind, home grown apples are often quite a bit smaller than those sold in grocery stores. (they taste better, though). It also looks like a young tree, which usually means smaller fruit.
