Yellow toenails can have several causes, with the most common one being a fungal infection called onychomycosis. This infection often leads to the nails becoming yellow, thickened, and sometimes brittle or crumbly. Fungal infections thrive in warm, moist environments like inside shoes, making feet a common site for such infections. Other causes of yellow toenails include aging (which changes the color and texture of nails over time), staining from nail polish (especially red, orange, or yellow shades), and underlying medical conditions such as diabetes, psoriasis, thyroid problems, or rare conditions like yellow nail syndrome. Nail trauma or repetitive pressure from ill-fitting shoes can also cause yellow discoloration. If the yellowing is accompanied by other symptoms or persists, it may be advisable to seek medical advice for proper diagnosis and treatment.