That dude does not know how to use binoculars.
Anyway, I don't know what you mean by "the colorama method", but there are a number of different ways to do sky replacements. Although, you do have to be careful. If you add a sunny sky when the lighting is diffused by a cloudy day, your shot is going to look weird.
On a basic level, you could do a garbage matte and a luma key, but you may have to do some rotoscoping for some of the other objects in the shot (building, wall, dude, etc.).
I'm with Szalam: this isn't going to be a simple deal to pull off. I also hope you're not thinking about replacing a cloudy sky with a clear-blue sky. It won't work visually. But a different, more interesting-looking cloudy sky? Yeah, that could work....