Old English geolu, geolwe, "yellow," from Proto-Germanic *gelwaz (source also of Old Saxon, Old High German gelo, Middle Dutch ghele, Dutch geel, Middle High German gel, German gelb, Old Norse gulr, Swedish gul "yellow"), from PIE root *ghel- (2) "to shine," with derivatives denoting "green" and "yellow" (such as Greek khlōros "greenish-yellow," Latin helvus "yellowish, bay").
Occasionally in Middle English used of a color closer to blue-gray or gray, of frogs or hazel eyes, and to translate Latin caeruleus, glauco. Also as a noun in Old English. Meaning "light-skinned" (of blacks) first recorded 1808. Applied to Asiatics since 1787, though the first recorded reference is to Turkish words for inhabitants of India.
Yellow peril translates German die gelbe gefahr. Sense of "cowardly" is 1856, of unknown origin; the color was traditionally associated rather with jealousy and envy (17c.). Yellow-bellied "cowardly" is from 1924, probably a semi-rhyming reduplication of yellow; earlier yellow-belly was a sailor's name for a half-caste (1867) and a Texas term for Mexican soldiers (1842, based on the color of their uniforms). Yellow dog "mongrel" is attested from c. 1770; slang sense of "contemptible person" first recorded 1881. Yellow fever attested from 1748, American English (jaundice is a symptom).