A gunsmith friend was messing about with metal finishes once and found that heating mild steel to about 200 Centigrade and quenching in soapy water did yield something that looked like faded case hardening colors.

To get back to aesthetics, it is arguable that a well aged shotgun, with faded case colors would be more attractive than an obviously restored and somewhat "forced" surface with renovated case colors. This is ofcourse is only an opinion from someone who positively detests case colors.