How do I fill a shape with color

The native Notes App is not a full painting/graphics App - and its capabilities in this respect are limited. If you are looking for a the ability to draw and colour-fill, you’ll need to identify a third-party App that suitable for you needs.

There are many art and drawing Apps in the App Store, of varying capabilities and complexity, so you’re sure to find something that meets your requirements.

Procreate, Vectornator and Affinity Designer are just three potential suggestions...