India has 4 festivals in this guide, spread across 4 cities. They run in March and December, and festival-day temperatures range from 24°C (75°F) to 32°C (90°F), with nights down to 8°C (46°F). That range is the reason there is no single "India festival outfit". The answer changes by month and by site.
3 of them run in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment.
The ground matters as much as the sky. You will find city streets, beach, desert and open field, which puts closed boots or trainers you accept will change colour at the top of the list for most of these events. 2 of the 4 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans costume, boho, water and party. Holi in Mathura is the largest of them at around 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.