I'm a professional photographer in Tucson. my suggestions depend on if you're doing an outdoor or indoor portrait. If you're looking for cheap, indoor studio photography (walmart, sears, etc), i would suggest that you all wear a similar color scheme. stay away from stripes or patterns. i've seen white shirts with jeans or khakis turn out nicely if the studio is using a white backgroud, but this is too stark if the background is darker. otherwise, darker solids are always more flattering and allows the attention to be on the faces (especially is the backgrounds are darker). If you are doing an outdoor shoot, i again would suggest darker, but you have more leeway. We recently did our family portraits outdoor and did a combo of blues and browns, everyone wore something different, but with complimentary colors (we all wore jeans) one of my boys wore a solid polo, the other two wore plaid shirts with tshirts underneath, all the shirts had the same color scheme, but weren't too matchy.
Hope that helps! if you have more specific questions, i'm happy to help!