Looking for black radiators, heard they never give out as much heat as the standard white ones, is this true?

To some extent, yes. Painted radiators do give out a little less heat compared to standard white radiators but the difference is minor. It will not impact the radiators ability to produce heat and heat a whole room. However, all of our colours are done by using a powder coating and so the colour makes no difference to the heat output. If you are considering painting a rad yourself, please not the more layers of heat resistant paint you add, the great the impact on performance.