blue

Blue is a color in the visible spectrum with wavelengths roughly 450 to 495 nanometers. It is perceived when light contains predominantly these wavelengths. In additive color systems used by light-emitting devices, blue is one of the three primary colors, alongside red and green; in subtractive color models used for pigments, blue is produced by specific pigments and is not a primary color.

Historically, blue pigments include ultramarine from lapis lazuli and cobalt blue; Prussian blue is a synthetic pigment from the 18th century. In nature, many blue appearances arise from structural coloration rather than pigments, such as the iridescence in butterfly wings and bird plumage. The sky appears blue because Rayleigh scattering favors shorter wavelengths, while the sea looks blue from selective absorption and reflection of sunlight.

Blue symbolism varies but commonly denotes calm, stability, and trust; it is also associated with sadness or melancholy in some cultures. It is widely used in branding, uniforms, flags, and fashion. The word blue derives from Old English blāw or blāwe and is related to other Germanic terms for the color.

In technology and design, blue is represented by the blue channel in digital color spaces such as sRGB and Adobe RGB. In printing, blue tones are typically produced by combining cyan and magenta inks. Blue has additional associations with temperature, described as a cool color in art and lighting contexts.