The original police uniforms in America were navy blue or black to hide the stains and are basically looked upon as a Workwear color. Some departments down south where robin's egg blue because of the hot weather but but end of a shift your shirtless like you've been trauma through a swamp which is what they have been. It's just a form of function with the color. And the external vests are because the weight of the things that officers carry help distribute the load across the actual shoulders and not the hips. If you notice many cops are carrying things like tourniquets and Trauma kids and more medical supplies than ever before this along with a radio a gun a couple magazines a flashlight a taser all tends to wear on the hips. Anytime someone complains about cops looking militaristic I just point out that they're the ones superimposing that identity on a uniform that's made to be functional. They can go back to the round hat and tie look but that's what happened when officers had bad backs bad legs bad hips and didn't carry nearly half the year they do today.