Gradient Tool

     If you wanted to select a certain part in your clip, in this case the sky; and changed its color, you can use the qualifier tool. But sometimes it's hard to use the tool because it makes the sky noisy or there are certain parts that you can't select.

original image

Gradient tool

     On your resolve, you can select the window tool located in the middle of the interface beside the qualifier button (eye drop icon). Then below you can see a white square. Select that and just drag it until the only part that you can see is the sky. Then you can adjust it to whatever looks or color you want.

this is how it will look like if you turned on the highlights (shift + 8). 

Here is the result.