Paint rollers in the real world are odd things - if you press hard with them they squash out the paint, and the texture of your canvas shows through. If you press more lightly they leave more paint behind. The roller isn't picking up paper colour, it's making your paint more transparent in areas.
If you want to have your airbrush strokes protected from reacting with the paint on the roller, airbrush on one layer, and use the roller on a different layer.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"