Crimson is a rich, deep red color, inclining to purple.^{[better source needed]} It originally meant the color of the kermes dye produced from a scale insect, Kermes vermilio, but the name is now sometimes also used as a generic term for slightly bluish-red colors that are between red and rose. It is the national color of Nepal.

Crimson (NR4) is produced using the dried bodies of a scale insect, Kermes, which were gathered commercially in Mediterranean countries, where they live on the kermes oak, and sold throughout Europe. Kermes dyes have been found in burial wrappings in Anglo-Scandinavian York. They fell out of use with the introduction of carmine, also made from scale insects, because although the dyes were comparable in quality and color intensity, ten to twelve times as much kermes is needed to produce the same effect as carmine.

Carmine is a slightly different shade of red, extracted from a different insect (female cochineal), although these denominations are sometimes confused or exchanged on purpose. Cochineal appears to have been brought to Europe by the Spaniard Hernán Cortés during the conquest of the Aztec Empire and the name 'carmine' is derived from the French carmin. It was first described by Pietro Andrea Mattioli in 1549. The pigment is also called cochineal after the insect from which it is made.

Alizarin crimson was invented in 1868. Alizarin (PR83) is a pigment that was first synthesized in 1868 by the German chemists Carl Gräbe and Carl Liebermann and replaced the natural pigment madder lake. Alizarin crimson is a dye bonded onto alum which is then used as a pigment and mixed with ochre, sienna and umber. It is not totally colorfast.

Several historical color models have described crimson as a basic color:

Crimson is a type of red. English dictionaries define crimson as a deep red, a rich red, or a purplish red. The word crimson refers to the kermes insect used to create the kermes dye. It comes from the Arabic word for the kermes insect, which was adapted to Medieval Latin and then Middle English, where it referred to both the insect and the dye c. 1400.

Earlier forms include cremesin, crymysyn and cramoysin (cf. cramoisy, a crimson cloth). These were adapted via Old Spanish from the Medieval Latin cremesinus (also kermesinus or carmesinus), the dye produced from Kermes scale insects, and can be traced back to Arabic qirmizi (قرمزي) ("red") [qrmzj] (listen^ⓘ), also borrowed in Turkic languages kırmız and many other languages, e.g. German Karmesin, Italian cremisi, French cramoisi, Portuguese carmesim, Dutch karmozijn, etc. (via Latin). The ultimate source from Classical Persian کرمست (kirmist), from Middle Persian; see Proto-Indo-Iranian *kŕ̥miš. Cognate with Sanskrit कृमिज (kṛmija). Doublet of kermes; also see carmine.

A shortened form of carmesinus also gave the Latin carminus, from which comes carmine.