Red is the luckiest colour in Chinese culture - happiness, prosperity, good fortune. At New Year, 红包 (red envelopes of cash) are handed out, doors get 春联 (red couplets), and brides wear red. Funerals use white instead.
usage
When 红 means 'popular' it works like English 'hot' - 他很红 = 'he's really popular right now', especially for celebrities and viral internet figures. The verb 走红 ('to go red') = to blow up / go viral.
register
Second reading 红 survives only in the archaic literary compound 女红 ('the feminine arts' - needlework, embroidery, weaving). You will essentially never see this reading outside that one word; everywhere else 红 is hóng.
history
The revolutionary sense comes from the Communist Party's red banner - 红军 (Red Army), 红卫兵 (Red Guards). During the Cultural Revolution being 红 meant ideologically pure; 红 is also a surname (rare today).
Left radical 纟 is the side-form of 糸 (silk thread). 红 originally named a specific pinkish-red dyed silk, and the radical fixes it in the textile-dye family: 紫 (purple), 绿 (green), 绛 (crimson). Colour names and silk thread were inseparable in classical script.
Right phonetic 工 supplies the sound (gōng → hóng, a velar-onset shift common in Old Chinese reconstruction). The archaic reading gōng in 女红 preserves the original phonetic value directly. Same phonetic series: 江, 攻, 功, 巩.
