The mixer drawer contains a set of mixing pots which you can use to mix all new colors to add to your creation.
You use the eye-dropper tool to suck up colors, and then squirt them into the mixing pots.
Colors can be sucked up from the paints in the paint drawer, from other mixing pots, or even from the picture itself. Click once to suck up a little bit of color, twice for a little bit more, ... up to 5 times to fill the dropper with paint.
Just like with real paint, the more you add of one color the stronger the effect of that color will be. For more details read the article on Mixing Colors.
Opening / closing the drawer
The mixer drawer can be closed by clicking it's button in the header.
When drawers are closed there is more space available for your coloring-in page.
To reopen the tool drawer just click its button again.
Scrolling through the items
To see additional mixing pots you can scroll up and down by pressing the scrolling buttons at the top and bottom of the drawer.