Yellow Toenails 

The most common cause of yellow discoloration in the toenails is a fungal infection. The fungus often develops underneath the nail, resulting in it becoming thick, raised and yellow in color.

Other potential causes for yellow discoloration of the nail include diabetes mellitus and lymphedema (chronic leg swelling). Yellow staining of the nails can also occur in individuals who use nail polish. A stained nail may take several months to grow out.