Assuming you are looking for a matt finish we can suggest Rust-Oleum Heat Resistant Paint 750°C. This can withstand sustained temperatures upto 650°c. 

 As we don’t know what actual surface temperature the storage heaters will get up to we can’t say for sure if the paint will be OK on any particular storage heater, it could depend on the model, type etc. You will need to check with the manufacturer to find out what the surface temperature of their particular heater will get to during use.