This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 黆 is uncertain. It combines 黄 (yellow) and 冘, but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, focus on the shape: 黄 (yellow) beside 冘, a component that appears in several other kanji. Remember it as a rare character with no common meaning.
For コウ, imagine a yellow (黄) coat (コート) hanging beside the right component: coat -> コウ, but the coat is just a mnemonic, not a real meaning.