Being a princess does not mean being a weak and fragile woman; on the contrary in fact. This is what Disney wanted to show through the colour of their clothes. You may not have noticed but most Disney princesses wear the same colour, be it Jasmine in Aladdin, Elsa in Frozen, Cinderella, but also Belle in Beauty and the Beast. They all wear blue, and if some are not wearing this colour, at least one of their accessories is this colour.
To be exact, they wear sky blue. According to Leatrice Eiseman from the Pantone Colour Institute, it is not a coincidence, rather on the contrary.
Sky blue has a meaning, beyond being just a simple colour; it conveys seriousness and reliability.
Thus, by dressing the female characters in this colour, which is stereotypically attributed to boys, Disney is giving them more power.