The visible spectrum, from 400 nm to 780 nm, determines the colours we see. Objects' colours are defined by the light wavelengths they emit or reflect.

Our ability to see colours relies on three sets of cone photoreceptors in the retina: sensitive to short (445nm blue), middle (545 green), and long (570nm red) wavelengths.

Congenital colour vision issues like deuteranopia, protanopia, tritanopia, deuteranomaly, protanomaly, and tritanomaly result from missing or altered sensitivity in these cones. Genes for red/green pigments are on the X-chromosome, while the blue pigment gene is on chromosome 7. Roughly 8% of men and 0.5% of women experience red/green defects, with deuteranomaly most common at 5% of men and 0.3% of women. Tritan defects, affecting blue-yellow perception, are rarer. Red-Green colour blindness is X-Linked Recessive.

Acquired Colour Defects:

Acquired colour defects span the entire  colour spectrum. Optic nerve diseases often cause red-green deficiencies, yet exceptions like glaucoma and autosomal dominant optic neuropathy lead to blue-yellow deficits initially. Using blue light on a yellow background in perimetry helps detect early visual field loss in glaucoma. Retinal diseases typically cause blue-yellow defects, but cone dystrophy and Stargardt's disease may mainly induce red-green deficiencies.

Clinical Testing of Colour Vision:

Clinical assessments of colour vision are designed to be conducted under lighting conditions resembling afternoon daylight in the northern hemisphere.

The Farnsworth–Munsell (FM) hue 100 test is a detailed colour vision assessment. It consists of 84 coloured discs, grouped into four sets of 21. The discs cover the colour spectrum, differing only in hue while keeping brightness and saturation constant. Patients arrange them in a specific order, revealing any colour vision issues.

The D-15 test involves colours from various parts of the spectrum and is used to determine if patients can arrange them in the correct order based on a reference colour.

The Ishihara pseudoisochromatic test plates are designed to specifically detect congenital red-green colour defects, the most common type of colour vision abnormality. These plates feature random spots of different isochromatic densities, with numbers or wavy lines represented by spots of varying colours. Patients with colour vision deficiencies will struggle to identify the correct numbers or patterns, as they can only be distinguished from the background by their colour, not by differences in contrast.

The Lanthony New Colour Test is a desaturated form of D-15. It assesses hue discrimination and can be useful in children.