On the right-hand side of your canvas, in the format panel, you may have noticed that there’s a tab called Style now. You’ll see the color schemes as well as other styling options that you can check or uncheck. You can mouse over each of the color schemes to preview how it will look with your diagram. Click o the little dots at the bottom of the color panels to see the next page of color schemes.
Leaving the options unchecked will give you a solid fill of color on your shapes. Checking Sketch will give the fill a hand-drawn look.