There are multiple reasons for darkening of the toenails. One of the most common reasons is a fungus infection that gets in under the nail and can cause them to turn different colors from black, yellow, green, or brown. Usually with a fungus infection, there is also significant thickening of the nail.
Other things that cause nail discoloration are improper shoe size and improper shoe type. When shoes place excessive pressure on any one of the nails, this can cause a small amount of microtrauma to the nail and the nailbed every step a person takes. Since the average person takes 8000 steps a day, every one of those steps is causing a little damage to that nail.
Toes that are particularly involved with this kind of discoloration are the 5th toes (the pinky toes). Because these are on the outside of the foot against the shoe, they are frequently more traumatized than the other toes. Another nail that frequently is discolored is the 2nd toenail, in someone who has a 2nd toe that's longer than their 1st (This is about 30 to 40% of the population).
Additional nail discoloration can come for medical conditions such as psoriasis, diabetes, and poor circulation. Luckily, many of these nails can be improved with simple and painfree treatments such as Laser, topical medications and replacement nails with Keryflex .