Yes, toothpaste can be used to whiten yellowed plastic solar lights. Simply apply a small amount of toothpaste to the surface of your solar lights and rub it in using a soft cloth or sponge. Rinse your solar lights off with clean water and dry them completely afterward. You may need to repeat this process more than once if the plastic is extremely yellow. The whitening process may also work on other colors of plastic but works best with clear plastic that has turned yellow.