What do the colored squares mean?

ChessUp’s patented AI assistance feature uses different colors to help you improve while you play. Each square can light up with 4 different colors: red, blue, green, and purple. What each color means depends on the level of AI assistance you’ve selected. In general, red is a mistake or blunder, blue is an OK move and green are the best choices. Purple is used to highlight the selected piece.

Assistance level 1 will show you all available moves, for each piece you touch, with no additional guidance. Assistance level 2 will highlight blunders and help you see mistakes before you make them. The assistance gets stronger as you increase levels up to level 6 which will show you the very best moves in the game! Try challenging a friend with each player using a different level of assistance – this is great for parents and kids!

See this article for more details: AI Assistance Levels