knee-pad made of red-dyed leather
韐 refers to a specific historical item: a knee-pad made of red-dyed leather. Its meaning is narrow and concrete, with no clear abstract extensions.
韐 combines 韋 (tanned leather) with 合, which likely contributes the sound. The character is a phono-semantic compound for a leather knee-pad.
The left side 韋 is leather, and the right side 合 suggests fitting or joining. Picture a red leather pad that fits snugly around the knee.
For コウ, imagine a leather knee-pad that is so well-made it could be worn by a 公 (こう, duke): 公 -> コウ, and the duke's red leather knee-pad is 韐.