How to replace a gray sky with a blue sky

There are always multiple ways to do anything!

And a comment paraphrasing Jarle's advice on sky replacement from his book ... is that a large part of successful  sky replacement is in choosing the right sky.

If the sun in the video is coming from one angle, the replacement sky needs to come from a clip at close to the same angle to the sun. Or it cannot "feel" right. Sky varies across the frame in color and brightness because of that relationship with the sun.

I have sometimes used this quick and dirty method. It's a bit of a hack - but hey! 

Draw a rectangle, choose blue linear gradient (blue to transparent blue) as the fill, and Darken blending mode. If it works, it works 🙂  If your shot is a moving shot you can add keyframes to position and rotation for the rectangle. You can of course combine it with masks with lots of feathering. 
I've totally overdone it in this example to make it obvious. Use your best judgement in your case. 

Hi Anisha,

Here are some steps to replace the gray sky with a blue sky in your video:

1. Add an Opacity Mask: Apply an Opacity mask to your video to isolate the mountains and a bit of the sky.
2. Track the Mask: Track the mask so it follows the camera movement.
3. Use Ultra Key: Add the Ultra Key effect and pick a sample of the sky for the Key Color. Adjust the Matte Generation and Matte Cleanup settings to get the best key. This will key some of the white in the water, but it will look natural.
4. Duplicate Clip: Duplicate your clip, remove all effects, and move it to Track 1.
5. Color the Sky: Select the duplicate clip, go to the Lumetri Color panel, and adjust the Temperature slider and RGB Curves to color the sky to your liking.

Hope this helps! Let me know if you have any questions.

Cheers,
Paul

Can be done using the Track Matte effect. Jarle Leirpoll has an example in his awesome book "The Cool Stuff in Premiere Pro" and I think also has an example on his website, premierepro.net.

The only way i can see this working is if you mask out the sky (this will be easier in after effects) and find some sky stock and track it to the movement. (again, easier in after effects) no easy fix for this, unless theres an ai out there.