Why does nail fungus cause discoloured nails?

A nail affected by fungus may look yellow, white, brownish or dull rather than clear. The colour can come from changes within the keratin, loosened nail material and debris collecting beneath or between layers. The nail may also become rough or thicker at the same time. Discoloured Nails is a very common presentation of Nail Fungus, although injury, psoriasis, staining and other conditions can look similar. Colour alone cannot identify the cause, so an examination is useful when the change persists or spreads.

Keep nails short, clean and dry without digging under them. Wear breathable footwear, change damp socks and avoid sharing files, clippers or towels. Clean and dry tools after use, and avoid applying polish over a changing nail until it has been assessed. A clinician may recommend a topical or oral approach depending on the nail pattern and any other health considerations. Do not stop a prescribed course simply because the colour has started to improve; nail growth is gradual.

Arrange care if a discoloured nail is painful, lifting, crumbling or accompanied by skin scaling. Prompt assessment is needed for redness, warmth, swelling, drainage or fever. A new dark streak, colour change after an injury, or pigment that reaches the surrounding skin also deserves medical review rather than self-treatment. If you have diabetes or reduced immunity, contact a clinician early for a changing nail.

What could be causing this colour change? Is testing needed before treatment? Should I stop nail polish, and what signs would show that the problem is affecting the surrounding skin?