Netherlands has 15 festivals in this guide, spread across 10 cities. They run in April, June, July, August and October, and festival-day temperatures range from 13°C (55°F) to 24°C (75°F), with nights down to 5°C (41°F). That range is the reason there is no single "Netherlands festival outfit". The answer changes by month and by site.
3 sit below 16°C, where the outer layer effectively is the outfit. 6 carry a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find industrial site, open field, forest, city park, beach and city streets, which puts waterproof boots at the top of the list for most of these events. 10 of the 15 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans camping, techno, underground and costume. Amsterdam Dance Event is the largest of them at around 450,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.