Using colour as a significant quality of a puzzle piece
Colour is a frequent visual quality of a puzzle piece. It gives distinct variation between pieces, providing both associative and dissociative aspects as necessary.
Colour is a part of our everyday lives; players do not need special training to appreciate this quality.
Colour has a particular caveat that individual players may suffer from various degrees of colour blindness. If you're trying to build an accessible game, with a variety of puzzles, it's a point to consider.
Beyond that, different mediums also render colours differently. For example, while something may be distinctive on a screen, it may lack contrast on paper. Items at different sizes, and under different lighting, also significantly impact how to distinguish colour.
It's important to always test the choice of colours in the environment in which they'll be used, and that test players represent a fair sampling of your audience.
Moreso, given the finiciky nature of colour, it's best to not rely on it as a sole quality for a puzzle piece. Other visual qualities like shapes and patterns, or relying on spatial placement for lights, can aid in distinguishing puzzle pieces.
If you have any questions, need an example, or want clarification, then let me know. Ask on Discord or Twitter.
Assume everything in this reference is a working draft, there's prone to be some mistakes and inconsistencies. I figure it's best to publish and get feedback rather than write for years in secret. The terms will change, the structure will shift, and the bugs will be chased out. It'll take a while.