I could tell the white balance was off because of the greenish tint on the coin. Also, the left side of the image looks green and the right side more red. This is due to the different color temperatures of the lights you are using. It's probably best to just use one light or if you're using two or more, use the exact same bulb in each of them. You can also use a white sheet of paper to kind of do the job of a second light.
When I said it was underexposed, I just meant the picture was too dark. If you are using your camera's automatic exposure, it will base it's reading on 18% gray. Your coin and the white piece of paper are much brighter (this is the same reason snow pictures often come out gray instead of white). If your camera allows for exposure compensation, set that to plus 1 and see if that helps. Play with that setting until your background looks white.