colorful
綵 centers on vivid, multicolored decoration, especially in textiles. The given meaning 'colorful' captures the idea of rich, varied hues applied to fabric or ornamental patterns.
綵 combines 糸 (thread) with 采, which likely contributes both sound and the idea of color or picking out hues. The character is associated with colorful silk or textiles.
The left side 糸 is thread, and the right side 采 looks like a hand picking colorful fruits from a tree. Imagine weaving those bright colors into a vibrant, colorful fabric.
For サイ, picture a sigh of admiration when seeing the colorful silk: sigh -> サイ, as you breathe out in wonder at the vivid threads.