The orange and red is notably more colorful than anything we see other Ocampa wear and compliments the dialogue that expresses Kes’s culturally controversial personality:
TOSCAT: You defied the Caretaker by going to the surface, Kes. Learn from the experience. Follow the path he has set for us.
KES: I’ve learned very well, Toscat. I saw the sunlight. I can’t believe that our Caretaker would forbid us to open our eyes and see the sky. Come with me. We’ll find your people.
She defies her upbringing in favor of seeing the sun, and helping the people who helped her.
In Cold Fire she has long sleeves beneath the puffed over-sleeves (in other appearances the under-sleeves are barely visible).
This dress she wears in the ‘future’ is clearly different, but it has the same long sleeves, and the over tunic is styled to call back to the more “standard” Kes look. I believe it is meant to be a more mature version of her sunlight dress.