Hong Kong has 1 festival in this guide, spread across 1 city. They run in March, and festival-day temperatures range from 23°C (73°F) to 23°C (73°F), with nights down to 18°C (64°F). That range is the reason there is no single "Hong Kong festival outfit". The answer changes by month and by site.
Conditions are moderate across the board, so the day-to-night temperature drop is the thing to plan for rather than the weather itself.
The ground matters as much as the sky. Every festival here is on city streets, which puts broken-in trainers or a low boot 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 indie, streetwear, urban and y2k. Clockenflap is the largest of them at around 60,000 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.