Unfortunately there isn’t a simple answer. The correct answer though, is something you feel comfortable in.
You don’t want to wear something that isn’t something you would normally wear. Eg if you are most comfiest in jeans and a simple tshirt – probably avoid a ball gown for your portrait session.
You also need to think about your setting, eg if we are doing a portrait session at the beach or the studio, we would choose outfits that go well with our surroundings AND time of year.
The studio is a perfect mix of casual and formal so you can bring a few options of outfits to portray this. While the beach is better for soft colours and light flowy fabrics.
For Autumn and spring, highlighting the respective colours is the ideal thing to do, with the warm oranges and browns for autumn and the pale pastel colours for spring.
What to avoid
Try not to match. Wear similar colours but please don’t all show up wearing exactly the same thing.