The short version
Dark, comfortable, and nothing you would be upset to see covered in dust. That is genuinely the whole rule. Everything below is detail on top of it.
You will be given a helmet at the base, so nothing on your head is needed. Everything else you are wearing is yours to choose.
Shoes
Closed or simply sturdier shoes are the sensible choice. Trainers are ideal — they grip the footrests and they survive dust.
Sandals are possible, and guests do wear them. You will just enjoy the ride more in something with a bit more to it, and you will not spend the afternoon shaking grit out of them.
Why dark beats white
This is an off-road route through mountain terrain, forest tracks and natural river-bed ground. Some days that means dust, some days it means mud and water. Nothing about that is promised either way — it is simply the reason light colours are an optimistic choice out here.
A spare T-shirt in your bag is never a bad idea, and neither is a towel if you are heading somewhere afterwards.
One thing that is not clothing
Sunglasses. There is no windscreen on a quad and the track gets dusty, so they do more work here than anywhere else on your holiday. Bring a pair you would not mind dropping.
The rest of the packing question — what to bring rather than what to wear — has its own page, because the answers are genuinely different and deserve the room.