I wondered this myself a while back, but after reading similar questions it would seem that a photocopy of the photo page of your passport will suffice.
You could also take a photocopy of your driving license and/or birth certificate for good measure to be on the safe side. You can also get them certified as a true copy if you want to be EXTRA safe (
http://www.postoffice.co.uk/document-ce ... on-service for example) but this is not necessary and won't be ground for rejection.
If you think about it, posting your passport to her could be risky and you may need it while her application is being decided. If you go to Pakistan to give it to her, then you can't travel back as the UKBA will have your passport!
Regarding color or black in white; I doubt it would matter, I'm sure black and white is perfectly acceptable as long as your photo and passport number etc are clearly readable on it.