loud (adj.) Look up loud at Dictionary.com
Old English hlud "noisy, making noise, sonorous," from West Germanic *khluthaz "heard" (source also of Old Frisian and Old Saxon hlud, Middle Dutch luut, Dutch luid, Old High German hlut, German laut "loud"), from PIE past participle *klutos- (source also of 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 (source also of Dutch luid, German laut). Paired with clear since at least c. 1650.