I am replacing a pull chain light fixture in my closet. The new ceramic fixture has a black, white and thin ground copper wire. The wires coming out of the ceiling are green and yellow. should the yellow be wired to the white and the green to the black wire?
Worldwide (except the former Soviet Bloc), the standard for safety ground wires is that they are colored Green, Yellow/green striped, or bare wire. All grounds are this, and grounds are no other colors.
Safety grounds are never, ever used for working conductors in a circuit. Since any circuit needs 2 wires, the ground must always be a third wire.
It is possible for yellow wires to be conductors, but cables aren't made in those colors, so that would imply the wiring to this location is in conduit (pipes optimized to hold wires). All conduits must end at an accessible junction box. So somewhere nearby would be the other end of that pipe. If you can find it, then wires can be added to the conduit as needed.
Also, make sure your lamp is not low voltage DC; those don't mix with mains AC power.