At the very bottom of the main window, are the master controls
Unlike the layer settings, master controls allow you to manage the final composition of all the layers mixed.
You can change the speed.
The background color.
The scale of all the layers.
You can apply color modifiers.
You can also change the transparency of all the layers.
Another interesting feature of the master control is the trace.
The trace can be used to apply a motion blur effect to the composition.
It is also able to paint with your layers simply by not clearing the background.