You can open the painting file you created from the free version with the full version. Then you can edit the paper settings for the layer, and make the paper transparent (set the paper opacity to 0%).
Then you're able to add a new layer, in the layer stack drag the new layer to below the original layer, and paint beneath your original single layer.
If you edit the paper opacity for the new bottom layer, you can be back to painting against a while paper colour generated from the new bottom layer.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"