Does the color/brightness of a dot on the ground mean anything

I know that a . is bare ground – as opposed to the green flora " etc… but I also notice that a . comes in many different shades of gray. Those shades of gray don't directly correlate to the "blueness" of the square they're in – and that blue background color definitely represents the tile's ambient light and thus my chance of being spotted. What, then, does the whiteness level of the . mean?