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Old English hlud "noisy, making noise, sonorous," from West Germanic *khluthaz "heard" (cf. Old Frisian and Old Saxon hlud, Middle Dutch luut, Dutch luid, Old High German hlut, German laut "loud"), from PIE past participle *klutos- (cf. Sanskrit srutah, Greek klytos "heard of, celebrated," Armenian lu "known," Welsh clod "praise"), from root *kleu- "to hear" (see listen).
Application to colors first recorded 1849. The adverb is from Old English hlude, from Proto-Germanic *khludai (cf. Dutch luid, German laut). Paired with clear since at least c.1650.