discolouration

Discolouration is the alteration of colour in a material or organism, resulting in a colour that differs from its normal appearance. It can involve darkening, lightening, or a change to a different hue. The phenomenon arises from chemical reactions, physical changes, or biological processes, and may be temporary or permanent. Discolouration can affect natural substances, manufactured goods, or living tissue.

In people, discolouration commonly refers to changes in skin or teeth. Skin colour changes can be intrinsic (genetic or disease-related) or extrinsic (sun exposure, inflammation, or injury). Conditions include hyperpigmentation, hypopigmentation, and pigmentary disorders such as melasma or vitiligo. Dental discolouration may result from ageing, dietary staining (coffee, tea, tobacco), medications, or trauma.

In materials, discolouration arises from oxidation, chemical exposure, moisture, heat, or UV light, often forming stains or patinas on metals, plastics, textiles, glass, or stone. Some changes are reversible with cleaning or coatings; others require remediation such as restoration, replacement, or protective finishes. In architecture and art, colour changes can be intentional (patina) or unwanted evidence of degradation.

Assessment typically uses visual inspection and colourimetric measurements; treatment depends on context, ranging from cosmetic or cleaning approaches to chemical stabilization or protective measures. Discolouration is a common indicator in diagnosis, conservation, and quality control, reflecting underlying processes and histories.