I recommend choosing comfortable clothing that makes you feel good, and beyond that, it only depends on the formality of your session. When it comes to color coordination, I’d recommend choosing from a pallet of colors, rather than strictly “matching” colors. For example, shades of blue and green, rather than everyone wearing blue jeans with a green shirt. If you’re looking for more or less formality, think about how you might dress for a comparable event — church picnic, family reunion, or a cousin’s wedding, for example — then apply a color pallet to that.