I don't understand how to use the layers – and how to apply colors to certain elements of my drawing.

Hey Jan,

Press the layer name, in Layers > Foreground, or press ctrl + 1/2/3.

You can also look at the guide for additional controls.

To set a color to the active layer, you can open the color picker with ctrl+p, as described in the guide — And type an hexadecimal color code.