I always wondered why those colors were chosen (white for neutral and black for line). I asked a professional electrician, and he replied "black means death", but I think that is just what he was taught as a safety mnemonic when an apprentice.
As I understand the US code, white (rarely grey) is reserved for neutral, green and green/yellow are reserved for protective ground, and any other color (typically black, red, and blue) can be used for line (especially in three-phase power). The normal European code has blue for neutral and brown for line, with green/yellow for protective earth.
Could be worse: in some European countries years ago, the color for protective earth (ground) was red--go figure.