The mystery has been solved!
TBH we didn't realise how often Disney Princesses wore the colour blue until we actually started to pay attention and there's actually a really cool reason behind that choice.
You're adding a bit of power to the character by giving her the blue
Leatrice Eiseman told Elle along with "It’s a very subtle way of saying, ‘Yeah, but young women, young girls, can be empowered, too." but along with the colour representing empowerment it also is a colour of hope and positivity by resembling the sky. "It’s something to look forward to, to see that blue sky, It’s dependable. It’s reliable. It might cloud up, but we know it’s there."
And ofcourse Emma Watson wore the famous Disney blue again in this year's 'Beauty and The Beast'. Costume designer for that film, Jacqueline Durran explained her reasoning behind it to Elle saying "There is a sort of refinement and crispness to light blue, but there’s also blue in workwear, It is a practical color, and a color that you can work in. In that sense, it is full of active strength."