Time for the lawyer to chime in...
A friend of mine went through a home invasion. She was home alone when the bad guys kicked in the door. She fired on them. Smith Model 15, blued, with Hogue rubber grips,aftermarket sights, a reshaped hammer, and a trigger shoe, loaded with .38 Special +P 158 grain LSWHP ammo. All six shots, then she reloaded. It was one of three ready firearms in the safe. The other two were a Glock 17 and a Bushmaster AR. She grabbed it because she was comfortable with the revolver. The cops didn't seize all of the guns from the house, though that revolver was taken as evidence. It was returned about a month later. Of the six shots she fired, four bullets were recovered from the door and door frame. One hit the windshield of their car, and one was not recovered, but blood was found in the driveway and in the car. No arrests. Nothing presented to grand jury. It's all done. The firearm used was never an issue, nor were the modifications.
If she had pulled out a pink AR with the kitty painted on the stock, nothing would have changed. Why? The circumstances of the shoot were that clear. A mother home alone when the door is kicked in and five guys run into the house? Hell, she could have used a select fire SBR and likely had the same results. Security camera footage backed her version of events. It was completely justified.
If you want to modify a purely range gun, knock yourself out. Want to paint the Punisher skull on it? Have at it. But don't use this as a defensive firearm. Keep it locked up. Want to modify your defensive firearms to make them more accurate, or rather to make you a better shot with that firearm? Go right ahead. Just don't bother with the cute saying engraved on it. I figure it this way...it might not hurt you in court, but it surely isn't going to help you.