WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Yes. There shouldn't be much of a safety issue involved, as long as you don't spray paint near an open flame. The manufacturers paint them black to help heat dissipation, and protect the metal from rust, as well as for appearance. Sand it to remove rust and smooth scratches before you paint, and tape off the leads.