Bathroom Light Fixture Wiring Question

So the painters uninstalled the light fixture in the bathroom, but didn't reinstall it. I bought a new one to go up there, but I am a little confused on what wires connect where. Coming out of the wall is 2 white wires that are capped together and then there is a yellow wire that is also coming out of the wall. The light fixture that I bought has a black wire, a white wire, and a copper wire. The instructions say connect white wire to white wire, black wire to black wire, and green/copper wire to green/copper wire. I don't have a black wire or green/copper wire coming out of the wall. So what wires connect to what?