Most people don’t realize that the color of your radiator shouldn’t be chosen solely based on the color of the room’s decor that the radiator resides in. But the color your radiator is painted also affects just how much heat it gives off too!
Darker colors radiate heat better than lighter colors, and a flat / matte finish radiates better than a shiny / glossy finish. This means your best choice would be a black matte color, while the worst would be glossy white.
Of course, not all decors will go with the black radiator. So you can expand on this by selected a dark color that still goes with your color scheme, whether it be a navy, forest green, burgundy, etc. But do keep in mind that the darker the better and the flatter the better.