What can commonly cause nail discoloration?

Nail discoloration can often be attributed to stains from nail polish or a fungal infection, making this answer the most comprehensive. Nail polish can leave behind pigmentation that changes the color of the natural nail, especially if darker polishes are used without a base coat. Additionally, fungal infections can cause nails to appear yellow, green, or even black as the infection progresses, leading to noticeable discoloration.

Other causes like direct sunlight exposure can lead to fading of color rather than discoloration, and while improper nail filing can damage the nail, it typically does not directly change its color. Using acrylics, while it can affect the nail's health if not done properly, does not inherently cause discoloration; rather, it may protect the natural nail from such changes when applied correctly.