Instructions
click mouse left button on "new block" icon (the very top icon)
click mouse left button at any place of the big gray area. In few seconds a white area will appear inside this gray area.
click mouse left button on "CP" (Color Picker) icon to choose the color.
You have to click on blue area to deactivate the color canvas
click mouse left button on "pencil" icon (the very botton icon) to choose the action (draw with pencil)
then, click mouse left button at any place of white area to start drawing. To stop drawing, just left click again. You can change the color at any time (stopping before...) just clicking "CP" button again and choosing another color.