This may seem basic, but this can show a lot about a person - especially if you take the time to think about the color. A bright color can
immediately give off a happy impression to the reader subconsciously. If the protagonist's favorite color is black or brown, there's already the idea implanted in the reader's mind that this character has a lot of problems that they face.
2. Skirt or Pants?
Although this usually applies to female protagonists, this can say a lot, not only about a character's personality but also about their body image. A character that doesn't feel sure of themselves might wear mainly pants, while an outgoing or confident character will wear a skirt every so often. It also creates an image in the reader's mind about what the character is wearing when you don't describe their outfit, so that they can assume that the character is wearing a skirt or pants depending on their previous preference.
3. Hair Styling
Do they hide behind their hair, or do they let everyone see their face? Is their hair short or long, maintained or disheveled? Do they have bangs? How do they use their hair to display emotions (when they twirl their hair they're nervous, when they flip their hair they're confident, etc.)?
Can a reader tell you these things about your protagonist? Let me know how successful you've been at letting the reader know your protagonist's style in the comments below!