[BrianG]

Yes;

yellow= 12v
orange= 3.3v
red= 5v

However, I would not use the chassis ground as the main output ground. Most of the time, yes, they are the same point electrically, but if the power ground is isolated from chassis ground (for regulator feedback for instance), the supply won't work right and may actually cause problems. Depends on the design of the PS...