There are a few factors that could be coming into play. The plants may be getting too much light, so you might want to first try moving the plants to the sides of your light table where the light is not as intense as in the center. If the light fixtures are adjustable, you can also try moving the lights farther away from the plants. Yellow or white foliage can also be caused by overly compacted soil or by room temperatures that are too high. If it's the former, repot the plant in the same size pot, removing as much of the old soil as possible without damaging the root ball. Replace it with fresh soil.
If you have a tiered light setup or a light cart, moving the plants to the bottom level often helps, because temperatures are cooler. The other possible problem is that the plant may be getting insufficient nitrogen. In this case, try a fertilizer rich in nitrogen (that's the first number listed in the three-digit analysis label on fertilizers). Usually, a well-balanced fertilizer for African violets is sufficient. Avoid purchasing fertilizers that contain urea, which hinders the violet's ability to absorb nutrients and water. You may also know that cold water on African violet foliage can discolor it. Usually the affected area turns beige and corky rather than white. Water the plant from the base or use room-temperature water.