Just chopped a cable, is red power, black ground?

red = +5 V
white = D-
green = D+
black = ground

(Page 89 of the USB 2 specification)

edit (after reading the other 649 pages)
OK, that's from the formal specification. Reassuring, isn't it? It can even get you an accepted answer. You can feel it coming: there's a but.

This is from page 94. At first sight it seems to confirm what I said, but then there's that word "typical". So not mandatory? I read some more, and the answer isn't clear. The word "typical" must be the most used word in the spec, and also seems to be used for mandatory specifications:

There's that word again, page 93. Sounds like non-binding, but the text above it does say "should be oriented to allow" (emphasis mine). So "typical" seems to be used for mandatory specifications. Talk about confusion!

Apparently there's only 1 way you can trust: measure it. Compare the wire color with the pin number on the connector. The pin number locations are shown in the drawing and their assignments in the table. I'm 99 % sure that it will agree with the "typical" wiring assignment.

There's just that other 1 %... :-(

Further reading
USB version 2.0 specification (.zip file format, size 19.5 MB)

It is a good bet, but I would not wage my life on it.

I've had cheap USB cables which had those colour wires connected to the pins differently to "the spec".

Better to "buzz it out" I think.