The argument type 'Color' can't be assigned to the parameter type 'MaterialColor?

primarySwatch is not a Color. It's MaterialColor. Which means it's a the different shades of a color a material app will use. Default color is blue. For more read articles https://dev.to/rohanjsh/custom-swatch-for-material-app-theme-primaryswatch-3kic

What is the difference between primaryColor and primarySwatch in Flutter?