It’s difficult (as in you need massive market recognition of which the specific colour is a major component), but you are allowed to trademark specific shades of a colour. Even then, like regular trademarks, you can only make the trademark in the specific field in which it is used commercially. In this case “Cadbury Purple” is registered in class 30, which covers most kinds of chocolate packaging. You can see their registration here.
It’s difficult (as in you need massive market recognition of which the specific colour is a major component), but you are allowed to trademark specific shades of a colour. Even then, like regular trademarks, you can only make the trademark in the specific field in which it is used commercially. In this case “Cadbury Purple” is registered in class 30, which covers most kinds of chocolate packaging. You can see their registration here.