Anna and Elsa are like two peas in a pod, as they are sisters, and yet are distinctly different from each other. Apart from the obvious changes in hair colour and clothes, there are more nuanced differences. These differences emerge from the two sisters' personalities - the warm, awkward and expressive Anna and the kind, proper and controlled Elsa. The question, how would these different personalities act in a certain situation, guides the character design.
On 'Frozen 2 Country Day' in October at the Disney Animation Studio, the filmmakers showed a glimpse of the number of iterations it took to get an angry Elsa and an angry Anna absolutely right. While the poised Elsa gets angry but at the same time is seeking a solution for the situation, younger Anna simply reacts to the situation and her expression is more undisguised. This is how the characters evolve and appear distinct in an animated movie.
The filmmakers have to endow the characters with a personality and stay true to the character through the film. In fact, the questions, what would Anna do and what would Elsa do, helped form the blueprint of the two characters. Not just their expressions, but also their choice of clothing reflects who Anna and Elsa are.
Elsa is magical, of course, and one of the perks of being such is that you can design your own clothes! Or so we think. So, Elsa's clothes are not made of materials you would traditionally find in Norway, where Arendelle is located. She weaves her dresses from the icy blue threads she spins; her clothes have a light, icy, magical quality. Elsa's clothing is also replete with motifs associated with ice. You will find traces near the bodice, at the hems.
Anna's clothes are designed to show her connection to her roots - the Kingdom of Arendelle. Her dresses have motifs associated with her home. The filmmakers have used these personality clues abundantly throughout the film. Remember these details when you watch the movie and you will learn much more about the characters than what simply meets the eye.