White balance doesn't mean getting white "white" as opposed to "grey" or "black," it means making neutral tones "neutral," not "orange" or "blue." It has to do with color temperature.
Any neutral object large enough to fill the frame will do (grey or white) since both would turn colors if there is a color cast to the scene. White balance is the process of removing (or rather compensating for) that color caste. The objective is to get red, green and blue channels equal, that is, a neutral tone.
If you use a yellow card to white balance, the camera will attempt to subtract the yellow from everything you shoot.