The easiest way to learn and write commands is not from the plugin but from your custom commands user file where you placed Watercolor. The reference docs are good but also take a look into @grosgood’s excellent tutorials. As everything is open source, be sure to read the gmic_stdlib.gmic and community files for syntactic and stylistic cues.
Lastly, post your specific questions and problems in G'MIC exercises for us to explore G’MIC collaboratively. Placing everything on a single thread makes it easier to search when you want to review or remember something.