Unless you’re wearing green nail polish, this is a color you don’t want to see on your toenails. It could be green-nail syndrome (chloronychia), which is caused by an infection. The culprit is usually bacteria that thrive in damp or wet conditions. Think hot tubs, sponges, even tight-fitting shoes that you’ve worn for a long time. The color is underneath the nail, so don’t try to scrub it off. Visit your doctor instead.