Below are some examples of the common causes for nail discolouration. Bear in mind that there can be overlap between colours and a mixture of internal and external causes too. Further information can be found here.
1. External Causes
These can manifest in a variety of forms and appearances. If the whole nail is involved it will usually have a curved border following the curvature of the cuticle of the finger.
a. White (leukonychia): In specks or lines the commonest cause is trauma. If the majority of the nail plate is involved, fungal infections are commonly implicated. Further information on Fungal infections can be found here.
Large areas of white discolouration from fungal Infection of the nail.
b. Yellow-Brown: Usually from external dyes and chemicals such as nicotine staining from cigarettes or hair dyeing. Fungal infections from candida also cause this colour change.
External External pigmentation from Silver nitrate, tobacco, henna, potassium permanganate.
c. Green: The bacterium Pseudomonas is very commonly implicated in this instance due to the nature of one of the chemicals it naturally produces bearing a green tint. It favours moist enclosed spaces and fingernails and nail plate boundaries commonly provide such harbour.
Discolouration from Bacterial Infections: Nail pseudomonas
d. Red-Brown streaks: These are think dark-red lines, looking like splinters of wood. The commonest cause is trauma and fungal infections. Psoriasis and occasional other rarer blood vessel inflammatory conditions have also been implicated.
2. Internal Causes
a. Brown: moles appearing in the nail matrix (the origin from where the hard nail plate surface grows) give rise to a straight streak of pigment that is propagated in the rest of the nail. In people with racially darker skin, this can be a common finding in multiple nails. Very dark, newly changing or irregularly pigmented streak may need medical evaluation to ensure there are no other untoward causes of it’s appearance.
b. Yellow-Orange: these usually manifest as spots in the nail. Psorisasis affecting the under-surface of the nail plate causes patches of inflammation that appears as ‘oil-spots’ under the nail.
c. Grey: usually this is due to a medication cause (most likely taken internally rather than applied on the skin). The appearance is slow and subtle in many cases and previously ingested medications are forgotten about by the time it’s noticed.
Drug induced pigmentation- Hydroxyurea