ORIGINAL: hands without shadows
It isn't always black and red, it can also be black and white, blue and yellow, orange and black, yellow and black, etc. The colours don't matter, what matters is that the darker wire is always negative.
EDIT: House wiring is AC, there is no positive or negative.
that's true, especially in automotive wiring
but the source power is always red and black, as in from the battery
you don't get into the other colors until power starts to branch
and house wiring is ac and there is no positive and negative per say
but my point was wire colors and how some are constant
green in house wiring is always earth ground
black is always hot and white is neutral