Does the gun have an actual red insert fitted into the front sight blade, or is the ramp just painted red? If the latter, just remove the paint. If the former, someone has damaged a scarce Colt and I would pass unless really cheap.
If someone installed a plastic insert then as the Judge said they modified a scarce model and more is the pity. Finding a replacement barrel without a modified sight will probably be impossible, and there isn't really a good way to repair the existing sight without eliminating what collector value is there.
Of course, we are approaching this gun as collectors. As mentioned if the price is right then it should make an excellent (although heavy! haha) shooter assuming everything else is in working order.