Paint large areas of the same colour with the paint bucket. Pick a color from the Color Palette then tip the paint onto the frame by clicking the left mouse button.
You can even change the color of Shapes (boxes, circles, polygons, stars etc.) and Text by using the Paint Bucket on them.
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Before you start on the beautiful details of your animation, try drawing a simple outline. This way, you can get the overall composition looking great, and you can make sure everything's going to fit in the frame! Once you're filling in the details, you can start deleting your outline if you don't need it any more.