There should be a little black box or square near the pencil image on the bottom of the screen. That is a drop-box or similar for choosing colours.
I checked on my account, and that is the way you do it. You will need to be on the white board or in a slide show for the bottom tool bar to show up on your screen. Also, you will have to give your students permission (make them a presentor) for them to have access to that bar.