Your toenails can tell you everything about your health. Yes, because when they become discolored they usually mean that they have picked up an infection or toenail fungus. This toenail discoloration means that your toenails have been stained by a product like nail paint or a product that have an allergic reaction. Sometimes, toenails may turn yellow as symptoms of something more serious like internal malignancies, rheumatoid arthritis, chronic lung conditions, lymphatic obstructions and more.