Yes they can age and change the exterior look. In some cases they turn a more slightly muddy color with a tint of brown while others (FoMoCo) ones can start showing some small chunks like Bakolite does. If the insides are still in good condition have seen some that polish then clear but that normally does not fix the off color look. Others have tapped off the spark plug wire connection points (inside) and applied a thin full coat of black epoxy. No paint would be applied inside the cap