dark grey, variegated, superlative
綦 centers on a mix of colors or qualities: a variegated pattern, a dark grey shade, or something extreme or superlative. The meanings connect through the idea of a striking, mixed appearance or degree.
綦 combines 糸 (thread) with 其, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with colored thread or variegated patterns.
The top 其 looks like a basket or winnow, and the bottom 糸 is thread. Imagine threads of many colors woven together in a basket-like pattern, creating a variegated, dark grey fabric that is superlative in quality.
For キ, picture a key (キー) that unlocks a chest of superlative, variegated silk threads: key -> キ, and the key opens the treasure.