I just took apart my 5" M&P9 Pro and noticed that the light blue painted end of the recoil spring is installed on the guide rod towards the muzzle end of the gun. From all the pics I have seen, the painted end is always shown installed on the guide rod first or towards the barrel lug. Does anyone know if it really matters or why the spring is painted on one end?

The spring is painted so folks at the factory know just what spring it is. There gotta be a dozen varieties of M&P pistols and several use different springs, hence the paint.

My M&P9 has the blue paint at the muzzle end.

-- Chuck

What really matters is that you install the spring with the hexagonal nut at the muzzle end.

Mine is the same as well.