Words for green, verdant and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Brythonic	*gwɨrð = green, verdant
Old Welsh (Kembraec)	guird = green, verdant, bluish green, grey guirdglas = sea-green, dark green, verdant, greenish
Middle Welsh (Kymraec )	guirt, gwyrd, gwyrt =green, verdant, bluish green, pale (blue), grey; fresh, lively; grassy gwyrddv = to become green, sprout, make verdent gwyrd(d) velyn, gwyrddvelyn = greenish-yellow, olive, light green gwyrddlas, gwyrdlas, gwyrdd-las = sea-green, dark green, verdant, greenish gwyrdloewet = bright green
Welsh (Cymraeg)	gwyrdd [ɡwɨ̞rð / ɡwɪrð] = green, verdant, bluish green, pale (blue), grey; fresh, lively; grassy gwyrdd gwyrdd(i)af, gwrddu, gwyrddio = to become green, sprout, make verdent gwyrddaidd = greenish, leek-green gwyrddfelyn = greenish-yellow, olive, light green gwyrddlas = sea-green, dark green, verdant, greenish gwyrddloyw = bright green
Old Cornish	guirt = green
Middle Cornish (Cernewec)	guirt, gwirdh, gwêr = green, flourishing
Cornish (Kernewek)	gwyrdh [gwɪrð] = green (not of plants or the sea) gwer [gwɛ:r / gwe:r] = green gwerdhu = dark green gwerem = emerald gwerik = greenish gwerlas = blueish-green, teal
Middle Breton (Brezonec)	guerz, guezr = green
Breton (Brezhoneg)	gwer [ɡweːr/ɡɥeːr] = green (artificial) gwerin, gweriñ = to green

Etymology: from Latin viridis (green, young, fresh, lively, youthful), from vireō (to be verdant, to sprout), from Proto-Italic *wizēō, from Proto-Indo-European *weys- (to increase) [source]. Words from the same roots include verdant, verdigris and verdure in English, vert (green) in French, verde (green) in Spanish, and verdhë (yellow, pale) in Albanian [source].

Proto-Celtic	*udnios/*utnio- = green (?)
Old Irish (Goídelc)	úaine = green, greenness
Middle Irish (Gaoidhealg)	úaine = green, verdant, greenness
Irish (Gaeilge)	uaine [ˈuənʲə/ˈwɛnʲə] = (vivid) green, greenness, verdue uaine éadrom = pale green
Scottish Gaelic (Gàidhlig)	uaine [uən̪ʲə] = green, greenness uaineach [uən̪ʲəx] = tedious Uaineach [uən̪ʲəx] = Green Party supporter, a Green uaineachadh [uən̪ʲəxəɣ] = turning green, being humbled uainead [uən̪ʲəd] = greenness uainealach [uən̪ʲəlˠ̪əx] = greenish sìor-uaine = evergreen
Manx (Gaelg)	eayney/geayney = green, greenness, verdure geaynaght = verdure geaynid = bile, greenness, verdure

Etymology: from Proto-Indo-European *wed- (green) [source]. Words from the same roots include [source].

Middle Welsh (Kymraec )	ehoec = heather-colour(ed), purple, green
Welsh (Cymraeg)	ehöeg = heather-colour(ed), purple, green, green colour hoeg = green

Etymology: possibly from *oec (heather) [source].

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionaire Favereau, TermOfis, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic