I have drawn this in illustrator:
It’s just a circle and a hand drawn line with the paint brush tool on top of each other.
How do I go about filling in on of the two halves with a color?
Let’s say you have these 2 shapes:
You select them and grab Live Paint Bucket tool:
Next, you just choose the color and click on the area you want to paint:
To get shape of filled colors you should use expand command.