Colour wheel, properties, schemes, symbolism, media, depth, RGB/CMYK, local colours

Colour is the visual sensation produced when light of different wavelengths reaches the eye. White sunlight contains all colours; Isaac Newton showed in the 1660s that a glass prism splits white light into a spectrum: red, orange, yellow, green, blue, indigo and violet (ROYGBIV). A rainbow is a natural spectrum formed by raindrops.

Why objects have colour

An object absorbs some wavelengths and reflects others. A ripe tomato looks red because it reflects mostly red light and absorbs most of the rest. A white shirt reflects nearly all light; a black cloth absorbs nearly all, which is why black clothing feels hotter in the Kano sun.

Light and pigment

• Light colour (additive): colours from light sources such as screens and stage lights. Primaries are red, green and blue (RGB); mixing all three gives white.
• Pigment colour (subtractive): colours of paints, inks and dyes. Each pigment subtracts (absorbs) some light. The artists’ traditional primaries are red, yellow and blue; printers use cyan, magenta, yellow and black (CMYK). Mixing all pigment primaries gives a dark muddy grey or brown, not white.

For WAEC and JAMB colour-wheel questions, assume the red–yellow–blue pigment system unless a question mentions light or printing.

Sources of colour for artists

Pigments come from minerals (ochres and earth colours, chalk, charcoal), plants (indigo, camwood, henna, kola, mango bark), animals (cochineal from insects) and synthetic chemistry (most modern paints). Dyes dissolve in water and soak into fibres; pigments are insoluble particles that sit on a surface held by a binder.

The eye and colour

Light enters the eye and falls on the retina, which contains two kinds of receptor cells: rods, which work in dim light and see only light and dark, and cones, which work in brighter light and detect colour. There are three types of cone, most sensitive to long (reddish), medium (greenish) and short (bluish) wavelengths. The brain compares their signals to produce every colour we see. This is why the light primaries are red, green and blue, and why colours fade to greys at dusk when only the rods are working: a market at twilight looks almost colourless.

Colour constancy

A white shirt looks white to us both under orange evening light and under bluish shade, because the brain adjusts. A camera does not; that is why photographs taken indoors under bulbs may look yellow, and why photographers set the white balance.