THE POSSIBILITIES ARE ENDLESS, BUT HERE A FEW THINGS TO CONSIDER
WHEN SELECTING YOUR COLOR SCHEME.
Season: Wearing light and breezy pastels while shooting in October with fall leaves in the background is not going to produce a great picture - you'll look out of place.
Patterns: A little goes a long way, when coordinating everyone's clothes. If you are all wearing flowers, polka-dots, checks, and plaid - it's going to get real ugly real quick. Just pick 2 or tone-on-tone patterns for interest.
Balance: Keep the colors and prints balanced equally.
Monochromatic: Wearing all black or white will make your pictures boring, flat and dull. Its fine to dress in one of those solid colors, but then you need to spice it up with accessories (jewelry, belts, shoes, etc.) Your entire family should not wear the same exact color, boring again.
Location: If you're planning on having your pics taken in a field, avoid wearing too much green - you'll blend in. Pops of green here and there are fine.