You need to make a mask to define which parts are metallic and which parts are not. Then use that mask to Lerp between different roughness and metallic values. Metallic materials give colored reflections, so, if you want the red paint to have a bit of colored reflections then give it 0.2 - 0.3 metallic value. It will also help boost the reflections.
Edit: I was late. Listen to Nick. 