Words for red in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*dergos = red, crimson
Old Irish (Goídelc)	derg [dʲerɡ] = red dergaid = to redden
Middle Irish (Gaoidhealg)	derg, derc = red, ruddy, bloody, sanguinary, red-hot, incandescent dergadas = to redden dergaid = to redden, make red, kindle, burn dergaide = reddish derge = redness, ruddiness, flushing dergthach = red, ruddy, hot, reddish
Irish (Gaeilge)	dearg [ˈdʲaɾˠəɡ / ˈdʲæɾˠəɡ] = red, rouge, red ink, glowing, raw, bloody deargadh = reddening, blushing, glow(ing), light(ing), bloodying, wounding, chafing, soil-turning, digging deargán = red matter or object, chafed, patch, glow dearglasta = blazing, red-hot
Scottish Gaelic (Gàidhlig)	dearg [dʲɛrɛg] = red, ruddy, flamming, red-hot dearg-amadan = bloody fool, complete idiot dearg-bhuidhe = orange dearg-lasrach [dʲɛrɛg l̪ˠasrəx] = flaming red, red hot deargachadh [dʲɛrɛgəxəɣ] = reddening, making an impression, blushing, drawing blood dearg-ruadh = bright red (hair, wool, etc)
Manx (Gaelg)	jiarg = red, sanguine, ruddy, bloodshot, rouge, gules, glowing, stark, florid, live, burning, abandoned, fair (intensive), turned (land) jiargey = chafe, sore, red jiargaghey = to blush, flush, glow, redden, blushing, glowing jiargid = grossness, reddishness, redness jiarg-bwee = orange jiarg-cheh = red hot jirgid = blush, glow, reddness, ruddiness

Etymology from Proto-Indo-European *dʰerg- (to dim, darken) [source]. The English word dark comes from the same root [source].

Proto-Celtic	*roudos = red
Gaulish	*roudos = red
Old Irish (Goídelc)	rúad [r͈uːa̯ð] = red, reddish rúadnaid = to redden forrúad = red, russet
Middle Irish (Gaoidhealg)	rúad = red, brownish, dark red, strong, mighty rúadnaid = to make red, redden
Irish (Gaeilge)	rua [ɾˠuə] = red, reddish-browm, russet, copper-brown; wild, fierce; rough, strong ruachan = reddening, rusting (vegetation ruachorcra = puce ruacht = redness (of hair) ruafholtach = red-haired
Scottish Gaelic (Gàidhlig)	ruadh [r̪ˠuəɣ] = (browny) red, rust-coloured, ginger, russet, red colour, redness; strength; red deer ruadhachadh [r̪ˠuə.əxəɣ] = reddening, making red ruadhan [r̪ˠuəɣan] = reddishness, overcooked good, tea which has been boiling all day ruadh-bhuidhe = auburn
Manx (Gaelg)	ruy [rɛi/riː] = red, ginger, tan, copper-complexioned, red-haired, foxy, sandy ruyagh = reddish ruyghey = bronze, brown, redden, tan(ned) ruyid = brownness, reddishness, redness, tan ruissagh = florid, reddish, rose-coloured, rosy, ruddy ruissaght = reddishness, rosiness, ruddiness
Proto-Brythonic	*rʉð = red
Old Welsh (Kembraec)	rud = red
Middle Welsh (Kymraec)	rud, rut, rhudd = red, ruddy rudit, rutaỽ, rudaỽ, rhuddo = to make or become red, crimson or violet rudell, ruddell = red, reddish rutem, rud em, rudem = ruby, garnet, ruby-coloured rudcoch, rruddgoch, rudgoch = red, crimson, purple, blood-red, bloody ruddlas, ryddlas = reddish-blue, purple, violet
Welsh (Cymraeg)	rhudd = red, ruddy, purplish, bay, tawny, brown, bloody, covered with blood, scorched, crimson, violet, purple rhuddaf, rhuddo = to make or become red, crimson or violet rhuddel(l) = red, reddish, brown, bay, red ochre, red colour rhuddem = ruby, garnet, ruby-coloured rhuddgoch = red, crimson, purple, blood-red, bloody rhuddiad = becoming red or pink, blushing, redness rhuddlas = reddish-blue, purple, violet
Old Cornish	rud = red
Middle Cornish (Cernewec / Kernuak)	rudh, rud, rûdh ruydh = red, crimson
Cornish (Kernewek)	rudh [ry:ð / ri:ð] = ginger, red rudh min = lipstick rudhek = robin rudhel = auburn, russet red rudhem = ruby rudhgogh = blood-red rudhik = reddish rudhlas = purple, reddish purple rudhlasdu = crimson rudhlos = greyish red, russet rudhvelyn = orange, reddish orange rudhwyn = gay, pink, reddish pink rudhya = to blush, redden
Old Breton (Brethonoc)	rud = red
Middle Breton (Brezonec)	ruz, ru, rus = red
Breton (Brezhoneg)	ruz = red ruzded = redness ruzdu = brown ruzellat = to glow ruzian, ruziañ = to blush

Etymology: from Proto-Indo-European *h₁rewdʰ- (red). Words from the same root include red, robust, ruby, ruddy, russet and rust in English [source].

Proto-Celtic	*kokko-/*kokki = red
Proto-Brythonic	*kox = red
Middle Welsh (Kymraec)	coch = red cochi, kochi = to make or become red, redden, brown, scorch, blush cochder, cochter, kochder = redness, ruddiness, flush kochdduy, coch-ddu, cochddû = reddish-black, russet, brown cochliw = red-coloured, bloody, redness, ruddiness kochwen = reddish-white, pink coghyn, kochyn, cochen = red-haired person, hare
Welsh (Cymraeg)	coch [koːχ] = red, redness, ruddy, scarlet, bloody; ginger (hair); brown; poor, feeble, unskilful cochaf, cochi = to make or become red, redden, brown, scorch, blush cochaidd = reddish, ruddy, bloody cochder = redness, ruddiness, flush cochddu = reddish-black, russet, brown cochfelyn = copper-coloured, reddish yellow cochlas = purple, violet, reddish blue cochliw = red-coloured, bloody, redness, ruddiness cochliwaf, cochliwio = to paint red cochrudd = crimson, ruddy cochwyn = reddish-white, pink cochyn = red-haired person, hare
Middle Cornish (Cernewec / Kernuak)	coch, couch = red, bloody
Cornish (Kernewek)	kogh = blood-red, crimson, scarlet

Etymology: from the Latin coccum (a scarlet berry), from the Ancient Greek κόκκος (kókkos – berry). This is also the root of coccinus (scarlet, scarlet-dyed), from which comes the English word cochineal, and related words in other languages [source].

Proto-Celtic	*wlannos = blood red
Old Irish (Goídelc)	flann = red, blood-red
Middle Irish (Gaoidhealg)	flann, fland [flan͈] = red, blood-red, blood
Irish (Gaeilge)	flann = (blood-)red, bloody, sanguinary, blood (poetic) flannbhuí = orange (colour) flanndearg = vermilion flannrua = blood-red
Scottish Gaelic (Gàidhlig)	flann [fl̪ˠaun̪ˠ] = blood (archaic) flann-dhearg = blood-red (archaic)

Etymology from Proto-Indo-European *welh₃- (to strike, hit, wound). Words from the same roots include vulnerable, valkyrie and Valhalla in English [source] and words for blood in Celtic languages.

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