In a nutshell...
ArtRage treats paper as separate from the paint applied on top. You have the wonderful ability in ArtRage to change the paper upon which you've painted without changing your paint.
So if you go into the paper settings panel for the layer, you can make your paper transparent, or change its colour or surface texture or whatever you like.
Because you can make your paper transparent, you can slide another layer of paper under your original painting and paint on a new piece of paper beneath.
To add a background layer beneath your original painting, first make the paper colour of your lowest layer transparent by editing its paper settings.
Then add a new layer.
In the layer stack drag the new top layer down to beneath your original layer.
Now you can edit the paper settings of the new bottom layer to make it paper-coloured.
And then you can draw to your hearts content behind the paint of your original layer.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"