There is no rule or 'right' paper color to use. It depends on the mood you want to create. The more you experiment and try different colors the easier it will become to make more intuitive color choices.
*TIP* If you aren't sure what color will work best with your subject, take some time to do some quick color studies on the paper choices to compare and contrast. These little 4x4 paintings helped me decide which color I like best for the bigger painting.