The easiest, quickest and cheapest way is to simply paint the top half of the er face of the front sight. There are gun sight specific paints to do this, or you can simply use a high quality enamel like Testers model paints.
The sight should still be available from S&W, call them at 800-331-0852 and ask. The sight height is measured from the top of the ramp to the top of the blade. You should have a gunsmith install the blade as it has to be drilled for the pin, which is a somewhat delicate job easily screwed up!
The third option is have a gunsmith install the red insert in your existing sight. There are kits available to cast an acrylic resin insert in-place in the blade.
If you try to do any of these yourself, except painting, the sight pin is .055" in diameter. You need a .050" cup point pin punch to remove and install the pin. The drill size to drill a new blade is a #54 AWG.
Mountain Guns are becoming quite valuable and there will probably never be any more made. I recommend painting your sight as this is easily reversible and will have no future effect on value, either for you or for future owners.
