Illustrator Live Paint
The Live Paint tool is an amazing tool in Illustrator that will allow you to color areas that aren’t objects.
Usually when you want to color an area, say a square, you can use the fill/stroke tool:

The downside is that you cannot color an area that isn’t an object, like an area between overlapping objects:

Luckily, we have the Live Paint tool that will do just that.
Start by selecting your objects.
Next, select the Live Paint Bucket (K) from the toolbar:

While holding the Paint Tool selected, hover over your selected objects and click to turn them into a Live Paint Group:

Once you’ve turned the objects into a live paint group, select a color from the swatches panel and click on the area you want to paint:

Final Results:

