For couple and group shots, it's best if you wear similar colors/tones. The idea is to minimize contrast in the clothing (individually and together). The eye is drawn to contrast, so if one of you is wearing white and another is wearing black, for example, the eye is drawn to the clothing, rather than your faces. You don't have to be matchy/matchy, but I'd pick something like everyone in warm colors or everyone in shades of blue/green. It's best to avoid stark white, but off white is ok.
For headshots and senior photos, solid colors work best for the same reason. Plaid, stripes or patterned clothing are distracting.
Of course there are exceptions. If we're doing a session in the winter and you want to have everyone wear funky sweaters, hats and scarves to showcase your family's fun personality or add color to the photo, that's fine. In this case the clothes become part of the theme and they should be highlighted. Similarly, if you're doing senior photos and you have a patterned dress you love and feel great in, by all means, let's get some photos.
These are some general guidelines, but they are not a hard rule.