Assuming this is an electric radiator we would recommend to consult the manufacturer as to what the maximum temperature the radiator and paint will need to be resistant too. 

Assuming it is not exposed to a dry heat of 90°C the previously painted radiator is clean, dry, sound and prepared as detailed in the technical data sheet you could consider Rust-Oleum CombiColor Original.

This can be mixed to many RAL, NCS or BS Colours in a satin or gloss finish. 

Please note this will give off an odour untill fully cured. 

We can offer Colour Sample Sachets in a generic paint to aid choosing a colour.