There are a few very uncommon causes that can cause your toenail to turn yellow.
The most common reason is nail fungus called Onychomycosis. This is a bacteria that is living of some of the properties of nails and is actually creating the color. You can get one toenail start to turn and then the fungus can move to your entire foot. You might also find that the fungus causes a single yellow streak up one toe.
The appearance is ugly, but fixing just the appearance doesn’t fix the problem. The bacteria is very hardy once it invades the nail. The best solutions include eliminating the bacteria from outside the nail, and from inside your body.