Origins
The word 'color' (or 'colour' in British English) entered Middle English around 1290 from Anglo-French 'culur' and Old French 'color,' from Latin 'color' (colour, hue, complexion, outward appearance). The Latin word is probably from an older form *colos, which many etymologists connect to 'cēlāre' (to hide, to conceal), from PIE *ḱel- (to cover, to conceal). If this widely accepted but not universally agreed-upon etymology is correct, 'color' originally meant 'covering' — the visible surface of things, their outward appearance as opposed to their inner reality.
This etymology carries a philosophical implication that the ancients seem to have felt: colour is a kind of deception. The Latin word 'color' was used not only for literal hue but also for 'appearance, pretence, pretext' — a rhetorical 'colouring' of the facts. English preserves this figurative sense in expressions like 'to colour the truth' (to distort it) and 'under colour of law' (under the pretence of legality). Colour, from its earliest attestable meaning, was understood as a surface that could deceive.
The spelling difference between American 'color' and British 'colour' is one of the most visible markers of the two standards. The '-our' spelling reflects the Anglo-French form 'colour' that entered Middle English. The '-or' spelling, advocated by Noah Webster in his 1828 dictionary, restores the original Latin form 'color.' Neither spelling is more 'correct' historically — one preserves the French intermediary, the other the Latin original.
Proto-Indo-European Roots
The Latin root 'color' was productive in both Latin and the Romance languages. French 'couleur,' Italian 'colore,' Spanish 'color,' Portuguese 'cor,' and Romanian 'culoare' all descend directly from it. The PIE root *ḱel- (to cover), if the connection is valid, produced a remarkably diverse family: 'conceal' (from Latin 'con-cēlāre,' to hide completely), 'cell' (from Latin 'cella,' a covered room), 'cellar' (from the same), 'helmet' (from Proto-Germanic *helmaz, a covering for the head), 'hell' (from Proto-Germanic *haljō, the concealed place), and 'occult' (from Latin 'occultus,' hidden). All these words share the core concept of something being covered, hidden, or enclosed.
The science of colour has a long history. Newton's 1672 discovery that white light is composed of a spectrum of colours revolutionized optics and challenged the Aristotelian view that colours were mixtures of light and darkness. Goethe's 'Theory of Colours' (1810) contested Newton on philosophical grounds, arguing that colour was a product of the interaction between light and darkness rather than a property of light alone. The modern understanding — that colour perception results from photoreceptor cells in the retina responding to different wavelengths of electromagnetic radiation — dates from Thomas Young's trichromatic theory (1802), refined by Helmholtz.
The metaphorical uses of 'colour' are extensive. 'People of colour,' 'colour bar,' and 'coloured' have complex and fraught racial histories. 'Flying colours' (success) comes from a victorious ship sailing with its flag (colours) still flying. 'True colours' (real character) derives from the same naval metaphor — a ship revealing its actual flag after sailing under a false one.