bright, brilliant
皥 centers on intense light: something bright and brilliant, like a dazzling white glow.
皥 combines 白 (white) with 臯, which likely contributes the sound and may relate to brightness or highness. The exact historical development is uncertain.
The left side 白 means white, and the right side 臯 looks like a tall, bright structure. Imagine a brilliant white tower shining in the sun.
For コウ, picture a bright white light shining like a 光 (こう, light): 光 -> コウ, and the whole character glows brilliantly.