Most cars in America have, at most, three colors of lights on the outside. There’s white for headlights and reverse lights, red for brake lights and rear lights plus, in some cars, amber for turn signals. Mercedes-Benz has just gotten approval to add a fourth color. Turquoise blue lights indicate when a Mercedes car is driving itself.
Blue is for priority vehicles, not just police.
Depends entirely on the state/jurisdiction in the US.
That is surprisingly not a federal law.
I thought it was part of the Vienna convention of 1968. But I just checked and the US aren’t a part of it.