If you have done it carefully, you should have a dark blue sky that blends to a very light color at the horizon and then back to a dark blue at the bottom. When your sky is dry you can go back to add the land and trees. They will be dark mixed with blue or green and brown. Mix several close colors on your palette and paint the trees and land. You can paint the trees lightly and then drip in some darker colors so they will blend like your sky did.
Watercolor artists practice a lot because they learn something with every painting.
Explore the Web John Constable was a famous British artist who used watercolors.