I couldn't find any C SDK sample for thumby color so I made one. This is a port of PICO-8 game "Hyperspace", a 3D space shooter game.
https://github.com/itsmeterada/hyperspace_thumbycolor (https://github.com/itsmeterada/hyperspace_thumbycolor)
(https://raw.githubusercontent.com/itsmeterada/hyperspace_thumbycolor/refs/heads/main/screenshot.jpg)