Dear Dave,
Does your monitor look something like the screenshot attachment? (see below)
If it does, it seems that the "monochrome" feature on your PowerBook has been switched on without you knowing, or it has happened somehow?
In either case, it's easy to get your colour back. Either press CMD+ALT+CTRL+8 and this will restore it. Or it can be accessed in the Universal Access System Preference Panel.
Hope it helps.