h₁reudʰ-

The root *h₁reudʰ- is one of the few colour terms that can be reconstructed for PIE with high confidence. The consistent preservation of both the form and the meaning "red" across all major branches is exceptional: most colour terms are relatively unstable in the lexicon and prone to replacement. Latin ruber ("red") and the related rubēre ("to be red") yield ruby, rubric (originally written in red ink), rubella, erubescent, and rouge (through French). The derivative russus ("reddish") gives russet and the country name Russia (from the Rus, possibly "the red-haired ones" or "the rowers," though the etymology is contested). Greek erythrós (ἐρυθρός, "red") shows the regular Greek treatment with a prothetic e- and the characteristic development of *dʰ to th. Derivatives include erythrocyte ("red cell"), erythema, and the Red Sea (Erythrà Thálassa, though the connection between the Greek name and the actual colour of the sea is debated). Sanskrit rudhirá ("red, bloody") preserves the full-grade vowel and the dental aspirate. The related word for blood, rudhirá (used adjectivally as "bloody, red"), connects the colour term to the physiological source of redness. English red (from Old English rēad, Proto-Germanic *raudaz) shows Grimm's Law: *dʰ > *d > t (voiced in West Germanic to d, then in Old English rēad). German rot, Dutch rood, and Gothic rauds continue the same line. The derivative rust ("the red oxidation") and the proper name Rufus (from Latin) extend the family. Lithuanian raũdas ("red, reddish-brown") and Old Church Slavonic rŭdŭ ("red") continue the Balto-Slavic reflexes. Old Irish ruad ("red," as in the personal name Ruaidhrí, "red king") and Welsh rhudd preserve the Celtic forms. The stability of *h₁reudʰ- across all branches contrasts sharply with other colour terms. Words for "blue," "green," and "yellow" cannot be reliably reconstructed to PIE, suggesting that the PIE colour vocabulary was structured differently from modern European systems — with "red" occupying a privileged, stable position.