reddish color, red, brown
緹 centers on a reddish-brown hue, often associated with silk or fabric. The given meanings—reddish color, red, and brown—all describe variations of this warm, earthy tone.
緹 combines 糸 (thread/silk) with 是, which likely contributes the sound. The character originally referred to a reddish silk fabric, with the color meaning extending from that.
The left side 糸 suggests silk or thread, and the right side 是 looks like a foot stepping forward. Imagine dyeing silk with a reddish-brown hue by stepping on berries or clay with your feet to extract the color.
For テイ, picture a tailor (テイ) carefully dyeing silk thread to the perfect reddish-brown shade: tailor -> テイ, and the silk takes on the warm color.