After looking through my photos, decide if you like the white or black backgrounds. I suggest if you like white or pastel (eg rugs on floor), then I suggest wearing a pastel colour. If you prefer the dark background I suggest wearing something dark. If you do like both backgrounds then simply have both light and dark clothing available on the day.
With newborn sessions I personally love natural skin tones with both, so either wearing a singlet or boobtube for the mothers and perhaps even shirtless dad. If that isn't your taste ,just follow as I have suggested above.
For families and couples I suggest wearing similar tones. Imagine you are all part of one outfit. If you were putting all the colours everyone is wearing onto yourself, would it be ok? I would suggest staying away from patterns as its an extra complication that you dont need. You dont have to match, but you need to compliment each other and not clash. Look through my photos and you will see how the families and couples complement each other.