Delete anything outside an area
If i have outlined an object, say a car, and i have added a red air brush around to give it a color, how could i remove all color outside the outline of the car? Masking etc. ?
You could make a duplicate of the outline you made of the car. Select the outline and the red air brush art. Open the Pathfinder palette and select Divide. Ungroup the selection. Delete the unwanted portions. Hope that helps.
You simply make a selection of the car (any way you choose = quick select/magic wand/pen path/ctrl click on the layer thumbnail/mask/etc.) and inverse the selection (ctr + shift + i).
The inverse selection will do just that: select everything outside of the current selection. In your case, everything except the car.
This can literally be achieved with 2-clicks of the mouse.