Wiring a wall light

I am wiring a sconce which has its own on/off switch. There is no fixture on the wall now -- just 4 wires sticking out of the wall with wire nuts on them. There are two black wires that are twisted together with a nut. A voltage tester confirms these are hot (I've only tested them together). Then there is a white -- presumably neutral wire -- which is twisted with a tan/gold color wire. Voltage tester shows no power to these wire. Question is, what is the tan/gold wire? Another neutral? Ground? How can I tell?