ordinary rice
粳 centers on a specific type of rice: ordinary, non-glutinous rice, the standard staple grain.
粳 combines 米 (rice) with 更, which likely contributes the sound and may relate to the idea of 'hard' or 'ordinary' rice, but the exact historical development is uncertain.
The left side is 米 (rice), and the right side 更 suggests 'again' or 'renew'. Picture ordinary rice being the staple you eat again and again, day after day.
For コウ, imagine a cook named Kou who always prepares ordinary rice: Kou -> コウ, and the rice is the plain staple he serves.