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 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.