Why the white frame on the rear sight of S&W revolvers

Rear sights on many S&W revolvers are available in all black, or with a white frame line around the cutout.

Was wondering (before I make it vanish with a black overlay) what is the reason for having this white line?

Seems to me that there are only two things to do with the sights - keep the tops of the front and rear sight level, and. keep the gap between the  rear sight and front sight equal on both sides.

I'm thinking that white line is just a distraction from what I need to see.  Am I missing something?

(The specific gun in this case is an old S&W Model 28, which after what seems like a lifetime, I'm finally comfortable with.)

(Hickok45 shoots this gun with either hand, single action or double action.  Most of the time he hits what he's aiming at.)

(And I should add that for years, I shot this gun "center hold".  I decided this past Monday I'm not good enough for that, so I switched it back to sub-6-o'clock hold.)



Sorry for the poor focus, but this is the best I could do with my iPhone.  I can improve on the white frame, but why?  I think it should be black.

Contrast for quick sight acquisition.

Primarily a reference for low light conditions.  Made little sense then, even less now.

Place the red block (front sight) in the white box and press the trigger.