@NigelG you mean to prevent the user picking a white color?
I’m not sure if thats possible but haven’t seen any color picker that prevents user from picking some colors.
In a particular use case though you can add some workflow that prevents your users from making some color combinations.
In the example you gave for instance you can add logic that prompts the user to pick a different color if they select the same text color as the button color.
Alternatively you can use the Palette colors feature of the plugin to define a list of colors that you want your users to choose from.