Cologne hosts 2 festivals in this guide, in North Rhine-Westphalia: Cologne Carnival, Summerjam.
Cologne has 2 festivals in this guide. They run in February and July, and festival-day temperatures range from 7°C (45°F) to 24°C (75°F), with nights down to 2°C (36°F). That range is the reason there is no single "Cologne festival outfit". The answer changes by month and by site.
1 sits below 16°C, where the outer layer effectively is the outfit. 1 carries 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 city streets and beach, which puts flat sandals you can rinse, plus trainers for the night stages at the top of the list for most of these events. 1 of the 2 has camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans carnival, costume, parade and party. Cologne Carnival is the largest of them at around 1,500,000 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.
