Filling colour in the terrain

Hi! I just wanted to know how I fill color in the terrain or better yet make something on it like a city below while the game is working in the sky. Can someone help me know how to do this?

 

Not from the description you've given. We need to know specifics about what you're doing. Ideally, tell us what you've already done, what the results were, and how you want them to be different, or where you got stuck.

If you just need to know how to work with Unity's terrain or put stuff in a scene then I suggest starting with the manual and/or the Learn section.