There's a feature request that's open on that, but it's not currently possible.
However, you can do the following as a work-around for the missing feature:
- Disable the "Auto-size canvas" option in the Canvas inspector.
- Zoom your document out: you want the canvas to be smaller than the screen.
- In draw mode, draw a shape that's larger than the canvas.
- Turn off the stroke for the shape and set the fill for the shape to whatever color you'd prefer.
Bit of a hack, but it should hold you until we get the ability to color the canvas into the app. Hope this helps!