★★★★★
Gentian violet is an antiseptic dye used to treat fungal infections of the skin (e.g., ringworm, athlete's foot). It also has weak antibacterial effects and may be used on minor cuts and scrapes to prevent infection. It will stain our toe nail (and everything else it contacts with...) but that is the working power behind it.
Apply with a Q-tip, allow to dye, put a sock on if you don't want your bed sheet with a purple streek on them from sleeping on them. Give it time, it does work.