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)