What speakers mean by it now
In British English a tricolour is, first of all, a flag cut into three equal bands, vertical or horizontal, each a different colour. France, Ireland, Italy, India, Russia, the Netherlands, and Belgium are the usual examples; more than twenty national flags follow the same layout. The word also works as an adjective for anything marked in three colours, including a dog whose coat is black, tan, and white. American English writes the same item as tricolor. A looser, less common sense still covers any three-coloured object that is not a flag at all.
The origin of the word tricolour looks, at a glance, like a home-made English compound. The pieces tri- and colour are both familiar, and that analysis is historically real at a deeper level. English did not, however, assemble the word from scratch. It borrowed it, then kept a spelling that makes the old Latin parts easy to see.
A Latin adjective, then a French one
Classical and later Latin already had tricolor, a third-declension adjective meaning simply three-coloured. It is tri- (from tres, three) plus color. The second half is older than the idea of a palette: Old Latin colos meant a covering, from a root also used for hiding and concealing, and only later settled on hue. French took the Latin adjective as a learned borrowing, tricolore (an older French spelling tricolor is recorded too). French historical dictionaries put the general descriptive sense in the 1690s. By the early eighteenth century the same stem was also a plant name, used of a three-coloured amaranth and then of a carnation, and it could describe a cat’s coat.
That double life, garden and ordinary description, matters for English. The Oxford English Dictionary treats the English word as of mixed parentage, partly from Latin tricolor and partly from French tricolore. Its earliest English evidence is from 1786, in the writing of the Scottish horticulturist John Abercrombie. That date sits comfortably next to the French botanical uses. Other desk dictionaries do not tell quite the same story. Merriam-Webster dates the English adjective to 1795 and the flag noun to 1797. Other notices put the flag sense in the same year, 1797, and delay the adjective until 1815. The disagreement is about which English use is attested first, not about the donor languages. A fair reading is that a descriptive or horticultural sense was available by the late 1780s, and that the flag noun, the sense that made the word famous, is securely placed in the 1790s.
How the flag sense took over
What fixed the word in public English was not a seed catalogue. During the French Revolution the national cockade and then the national flag were called, in French, a three-coloured banner. French sources attest that flag use by 1791. The vertical blue-white-red arrangement of the modern French flag is dated to 1794. English borrowed the noun in that political weather: a flag of three colours in nearly equal masses, first and especially the emblem of revolutionary France.
From that point the meaning widened without dropping the French association. A tricolour could still mean the French flag in particular, but it also became a ready label for any national flag built on three equal bands. Italy, Ireland, India, and others adopted the design type, and English kept the French-derived name both for the type and for the French instance. In French the noun later grew senses that English never fully copied: a supporter of the national colours, and, in twentieth-century sports writing, a French athlete. English did pick up a specialised animal sense, above all for a three-coloured dog.
The spelling split follows the older colour / color divide. Colour had been the usual English form since the fourteenth century, by way of Anglo-French. A classical respelling color became the American standard. Hence British tricolour beside American tricolor. Both remain transparent: anyone who knows the prefix and the colour word can parse the whole without knowing French. The English form can therefore be described both as an anglicisation of French tricolore and as the transparent sum of tri- plus colour. Those are two descriptions of the same history, not two rival etymologies.
Relatives, and a leftover from the garden
Cognates make the path easy to follow. Italian tricolore, Spanish and Portuguese tricolor, Catalan tricolor, and Romanian tricolor all continue the Latin compound, often with French as the immediate model for the flag sense. German Trikolore is a borrowing of that flag word. Dutch driekleur is a calque rather than a loan: it says three-colour in native pieces. Russian триколор is a later international borrowing of the flag term. None of this is folk etymology. It is the same two morphemes moving through different spelling systems.
The Latin adjective never meant a flag. It meant three-coloured. That older sense is still alive in English — film stock, a coat, a flower — but it is no longer the headline meaning. The headline meaning is heraldic and political: three bands, three colours, one national claim. If a single dating is worth keeping, it is the horticultural first date in the OED. Before English newspapers needed a name for the French banner, a gardener already had a use for a plant in three colours. The Revolution did not invent the compound. It specialised it, and that specialised sense is the one most speakers now reach for first.