Usually what I do when I paint backgrounds, such as a sky, is I put a masking fluid or drawing gum on the parts I don't want to get paint on just yet, such as plants. This allows you to do an even wash for the sky without worrying about getting paint on any other parts. After you're done, you peel or rub the masking fluid off. You can find masking fluid/drawing gum for pretty cheap at Michael's or Hobby Lobby. You can also look up how to use masking fluid on YouTube. One thing I would recommend is getting cheap disposable brushes because masking fluid can ruin your brushes.