Compound ideograph: 大 (big person) above 火 (fire). The encoded scene: a person standing before a great fire, their face glowing red in the firelight. The word "red" emerged from this universal human image — flushed skin tones in front of flames. Identical across 繁體 / 新字体 / 简体.

Critical Mandarin/Japanese vocabulary split: — Mandarin has TWO words for red: 红 (hóng, the everyday word) and 赤 (chì, formal/literary). 红 dominates daily speech: 红色 (hóngsè, red), 红茶 (hóngchá, black tea — "red tea"), 红包 (hóngbāo, red envelope cash gift), 红绿灯 (hóngludèng, traffic light). 赤 reserved for formal/idiom use: 赤道 (chìdào, equator), 赤字 (chìzì, deficit), 赤裸裸 (chìluǒluǒ, stark-naked / utterly exposed). — Japanese uses 赤 (aka) as the everyday word for red — there is no equivalent of 红 in modern Japanese kanji. 赤 (aka) is on every traffic light and Japanese flag description.

Mandarin: chì, falling 4th tone (literary). 赤道 (chìdào, equator), 赤裸裸 (chìluǒluǒ, naked / exposed), 赤字 (chìzì, financial deficit). The word 赤 alone in modern Chinese feels classical.

Japanese: on-reading セキ (seki) for compounds — 赤道 (sekidō, equator), 赤十字 (sekijūji, Red Cross), 赤外線 (sekigaisen, infrared rays). Less common シャク (shaku) in 赤銅 (shakudō, copper-gold alloy). Kun-reading あか.い (aka.i) is the everyday adjective — 赤い (akai, red), 赤ちゃん (akachan, baby — "the red one", from a newborn's flushed skin), 真っ赤 (makka, deep red). 赤ちゃん is one of the warmest words in Japanese.

The Japanese flag, 日の丸 (Hi no Maru), is white with a single red 赤 disk — 日章旗 (Nisshōki).

Memory aid: person + fire = red flush of firelight.