Assuming that you've drawn the circles and lines using the Ellipse and Line Segment tools (or the Pen, etc.), the colouring is relatively simple using the Live Paint Bucket or the Live Paint Selection Tool (Illustrator CS4 and newer). These tools require that the shapes and lines are not grouped or placed in different layers (if they are grouped, ungroup them first: select, then press Cmd-Shift-G).
Select all shapes and lines that belong to the diagram using the normal Selection Tool, or press Cmd-A (Windows: CTRL-A).
Click on the
Live Paint Bucket in the tool palette. (It might be 'hidden' under the 'Shape Builder tool' – click on it and hold the mouse button down to access the hidden items. You can also have the group of tools open in a separate tool set.)
Then move the mouse over the selected group of elements and click once to turn it into a Live Paint Group object. The tool will then interactively highlight in red the area that it recognises as an individual painting area.
To select a colour, the Live Paint Bucket is connected to the Swatches palette. Use the arrow keys to navigate and choose a colour… Or mix your own colour in the Colour palette… When done, click in the area to fill it with the colour.
It's also possible to use the
Live Paint Selection Tool to just select an area within the elements by clicking it, and then paint it by clicking on the colour in the Swatches or Colour palette.
(Applying the Live Paint Bucket compounds the selected elements into one 'Live Paint Group'. Individual elements can still be selected using the Direct Selection tool, but to completely revert it into unpainted – e.g., to make structural changes – choose
Object > Live Paint > Release.)
(Unfortunately the tool's mouse pointer isn't recorded correctly in this screenshot for some reason.)