Hi, Mayor. That's a good question - I thought rank had to do with those shoulder things, some have 1 bar, some have 3 bars, etc.
Maybe the traffic women kept being accidently run over and they are changing to white uniforms to be more visible - but NK can't afford to replace them all at once, so they are replacing them as they can afford them??
Also some times the blue uniform women wear white hats, other times blue hats. But I've never seen a picture of a white uniform woman wearing a blue hat. Don't know if that means anything.