Whiteboard

Press Shift+Cmd+W to fill the screen with a solid board and draw on that instead of over your work.

Press it again to go back to the transparent overlay. Anything you drew is still there.

Use it when what you are explaining is not on screen. A diagram, a bit of maths, a rough plan, and you do not want whatever happens to be open behind it showing through.

Changing the colour

Open Settings (Cmd+,) and set the board colour under Whiteboard. It changes straight away, with no restart.

A dark board with light strokes is easier on the eyes on a call that has been going a while. A white board photographs better if people screenshot it.

Drawing on it

Every tool works exactly as it does on the transparent overlay. See Annotating.