Some options:
1. If you want the most authentic, original S&W insert, buy a new front sight blade with red insert. Carefully push out the insert from one side and replace with the new one. There's two bumps on the bottom of the red insert. They'll be sheared off so just use a tiny dab of super glue when you push in the new insert.
You don't want to replace the front blade or blade and base. The pins are polished flush and then the barrel is blued. There's no way the work won't show.
2. Do as Muley Gil suggests.
3. Try a plastic rejuvenator usually found in auto parts stores for automotive applications; there's some amazing products made for all the plastic on car exterior trim pieces. Or soak your faded sight with Armorall.