ArtRage uses a full 3D lighting system to light the canvas and paint surface. The canvas surface is roughened and the light transmission across the surface is cast directionally from the upper-left. So the resultant surface will no appear to be uniform white.
It makes the canvas feel more natural and less digital. In the real world you will never see a pure white surface - there will always be the effect of lighting and your environment on your surface.
For people wishing for a more digital experience, there is the option to turn off the 3D lighting by pressing the <F5> key.
Regarding the vertical smears when you're in the new painting dialog: That's actually a vertical blur of what's behind the canvas. Fill your canvas with bright colours, then open the new painting dialog and you'll see the colours smeared behind the dialog. It's a way to draw your attention to the dialog by distorting the detail behind.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"