Hi Frank,
Two things to check. First click on the Image menu and pull down to Mode and verify that the document is not listed as Greyscale or Bitmap.
Once you verify that the document is a color document (with I am assuming that it still is) you can then check to see if you have turned off Channels.
Try using the Command ~ keyboard shortcut (Control ~ on Windows) and see if the color doesn't magically reappear.
Not that I am speaking from experience or anything...
In Adobe products, Command 0 makes the window fit on screen, and in programs other than Photoshop, Command 1 makes it actual size. In Photoshop, Command 1 shows the Red Channel only. (Command 2 is the Green Channel, Command 3 is the Blue Channel and Command ~ is all Channels.)
*** Now after I wrote this, I just found out that Adobe corrected this in CS4. Now Command 0 makes the window fit on screen and Command 1 makes the window actual size. ***