色 originally meant the look on a face — early forms show one figure above another, reading expressions. From facial color it broadened to color itself: what shows on the surface.
色 completes every color word: 红色, 黑色, 绿色 — color + 色 is the pattern. It kept its 'expression' sense in 脸色 (facial complexion — 看脸色 means reading the room via someone's face). Confucius warned against judging by 色 (appearances); Buddhism's Heart Sutra declares 色即是空 — 'form is emptiness' — making 色 one of the most philosophically loaded characters a beginner will casually use for 'blue'.
What shows on the face — expression, and every COLOR.
色即是空 'form is emptiness' — the Heart Sutra's most famous line uses the same 色 as your favorite color.