yellow, surname
黃 centers on the color yellow. The surname sense is a natural extension of the color name used as a family name.
黃 is the traditional form of 黄. It is thought to depict a person with a jade ornament or a field with a distinctive mark, associated with the color of earth or jade. The exact origin is uncertain.
The shape suggests a field 田 with something spread across the top and bottom, like yellow earth or a yellow ornament. Think of 黃 as the rich yellow of soil or jade.
For コウ, imagine a yellow coat hanging in a field: coat -> コウ, and the coat is bright yellow like 黃.