There is no specific dress code for the tour. You can wear anything you prefer. But here are some recommendations. It is better to wear loose, comfortable clothes, especially if you take a summer trip.
When it comes to footwear, high-top boots are the best. Even open-toed shoes will be better than low-top tennis shoes. When you walk the dunes with those low-top shoes, you will frequently find yourself removing the boots to get rid of the sand. Be sure to bring your shades, sunscreen, and hat with you. Keep jewelry to a minimum and keep a jacket handy during winters as the temperature falls rapidly after sunset.