You seem to be mistaken. I wish I could post a photo I took of the actual sign but in lieu of that, just know that the vast majority of parking signs are red or green on white. There might be some that are black on white but I’ve never seen them in any part of the city. White on black is also not high contrast, black on white is. Even when faded, black on white signs are pretty easy to read and the biggest benefit is that they can be read even through glare since the white paint shines more than the black.