There are a lot of different conventions, actually. For example, in US house wiring (120v 60Hz AC single-phase), black is always hot, white is neutral, and ground is green, green with yellow stripe, or bare wire. This is from the National Electrical Code, which is not a law but is a set of conventionms adopted by most or all states. (Red and blue are used in 2- or3-phase power for Phase s 2 and 3.)
But of course, an amplifier is not a house. :^)
In industrial DC power work, a common convention is red for positive, black for negative and ground is white. (I don't know whether electric cars follow this or not ...) For tube circuits, negative is usually called ground though it may or may not be connected to a true "ground;" black is commonly used.
You can see the confusion - black is hot in house wiring, negative in DC power, and ground in tube amps. And what colors might be used for internal wiring, e.g. filament power? Or AC signal with a DC bias vs. AC signal with no DC bias vs. DC bias with no AC signal? There are really no useful standards. We use a small number of colors, where needed, to distinguish one wire from another. It's often red for positive power and black for ground, but otherwise quite arbitrary.
