From a 2015 interview by Drew Taylor with directors / writers Jennifer Lee and Chris Buck:
Question: Can you talk about developing the look of "Frozen Fever"? It's kind of fun to see the kingdom with a springier color scheme.
Chris Buck: We got a lot of our team back together to do the film, so our art director was back and we gave it that "Frozen" look. He's great with color and design so we wanted to stay true to the original and Arendelle. So it's pretty much Arendelle set in the summertime and how beautiful that could be. That was it. Instead of the wintery color scheme from the movie we got to play with more summery stuff.
Bryan Alexander quotes Lee in 2015 USA Today:
"That was a lot of fun, to design a dress that Elsa would make in the summer, but still be true to herself being made of ice," says Lee.
Very interesting is Shan Li's 2015 LA Times article Disney works to keep its hit film 'Frozen' hot. It contains information on merchandise and such and shows how important it was ("in November, Disney said it had sold 3 million role-play dresses."). Furthermore:
Michael Giaimo, art director for "Frozen" and production designer for "Frozen Fever," said employees from the creative side talk with retailers and Disney's consumer products side throughout the design process.
"Sometimes there is an initial conversation even before they see any of your designs," he said. "They'll say 'We know girls like this and girls like that.'"
But ultimately, the top priority was creating artwork that served the story, Giaimo said. That included avoiding the color pink in the movie, except for a deep magenta on Anna's cape, he said.
"I wanted to stay away from a Barbie world," he said. "The sisters were coming off so strong and distinctive and provocative — I didn't want them to be associated with a palette that is so out there in the world in every way."
In conclusion, the colors were chosen because Frozen Fever takes place in summer. Anna's and Elsa's dresses should also reflect their personality, i. e. "strong and distinctive and provocative".
The Disney Wiki entry on Frozen Fever contains the following
According to Jennifer Lee, Elsa wears a green dress for much of this short (using her magic to turn her signature blue dress green) specifically to dispel the perception held by fans that Elsa consciously only ever wears blue.
However, no source is given and I didn't find the respective interview yet. So take this information with a grain of salt. I include it here for the sake of completeness.