红 is a simplified form of the traditional character 紅, which is a phono-semantic compound of semantic 糸 “silk” and phonetic 工 (gōng 🔊), “labour”, “work”. Something to do with (textile) workers’ movement and, by extension, revolution, communism? Not exactly (although 红 grew to represent all that too). Lawrence J. Howell explains in his Etymological Dictionary of Han/Chinese Characters:
彔 thread + 工 here an abbreviated form of 汞 (gǒng) mercury → fabric/textiles dyed a deep red (compare 絳) → crimson; rouge → lipstick.I presume that “mercury” here refers to cinnabar (mercury sulphide), a historic source for the red pigment vermilion. According to Wikipedia, in China cinnabar has been used for its colour since as early as the Yangshao culture (around 5000 BC to 3000 BC).
Interesting compounds of 红 include
- 口 + 红 = 口红 (kǒuhóng): lipstick
- 月 + 月 + 红 = 月月红 (yuèyuèhóng): China rose (Rosa chinensis)
- 红 + 人 = 红人 (hóngrén): favourite; pet; rising star; celebrity
- 红 + 花 = 红花 (hónghuā): safflower (Carthamus tinctorius)
- 红 + 茶 = 红茶 (hóngchá): black tea