Heig-boy is a Game Boy emulator made during my studies with 4 other people. It is written in C, but the code has been made to be easy to understand. If you want to know a bit how an emulator could look like, then this is probably a better idea to start looking at Heig-Boy, rather than VisualBoyAdvance for example

Heig-boy features ColorIt functionality, which allows gamers to colorize their own games, with a script-based system. This system has been refined for the occasion. A new parser has been written and included in Heig-Boy, and VBA CE has been updated with it. Finally, a GUI to colorize your games more easily completes the collection.

On this page, you will be able to download Heig-boy and find resources about ColorIt, including a documentation on how to implement it in your own emulator, with some samples and the pre-written parser.