Hue is the dimension of color that is determined by wavelength. It is what we name when we say red, green, blue, or yellow.

Visible light ranges from roughly 400 nanometers (violet) to 700 nanometers (red). The visual system extracts hue by comparing the relative activation of the three cone types, each tuned to a different portion of the spectrum. Hue is distinct from brightness (the perceived intensity of the light) and from saturation (the purity of the color). A pure spectral light has high saturation; mixing in white or gray reduces saturation while leaving hue roughly constant.

Color naming systems and color spaces organize hues into circles or other continuous arrangements, reflecting the fact that hue is a circular dimension—red is closer to violet than either is to green. Cultural and linguistic differences affect how finely hues are categorized, yet the underlying sensory continuum is shared.

Hue is the qualitative aspect of color that survives changes in lighting intensity and that allows us to distinguish ripe fruit from unripe, signal from background, and one object from another.