Colombia has 2 festivals in this guide, spread across 2 cities. They run in February and March, and festival-day temperatures range from 20°C (68°F) to 33°C (91°F), with nights down to 9°C (48°F). That range is the reason there is no single "Colombia festival outfit". The answer changes by month and by site.
1 of them runs in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment. 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 open field, which puts waterproof boots at the top of the list for most of these events. None of them have camping on site, so you are travelling in and out each day and the outfit has to survive the journey at both ends.
Stylistically the crowd here leans carnival, parade, costume and tropical. Barranquilla 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.