You'll notice in the before and after shots below how many times the sky was not blue when I started. Changing the weather is challenging enough as a colorist but when you add in clouds of colored chalk pitching back and forth across the sky, it introduces a whole new set of problems. There are lots of different ways to tilt an overcast sky. The simple route is to select it and push everything to blue. But when the sky is white and the balloons are white like in one of the shots, you've got to play with how far to push before it gets really really noticeable. Honestly the biggest thing you're doing when changing wheather is working as many corrections as it takes to cover your tracks and make it as natural as you can without drawing attention.
It doesn't always work as well as you'd like it to. Often you end up pulling it back farther than youd like to keep edges clean, but overall it turned out well and gave it the vibe it needed. Enjoy.