Hi all i’am new on the forum 
On the internet i have seen a video where a guy pressing the keys SHIFT + CTRL remove polygons from a 3D model
you can see the video here
The problem is: why in the video the dragged zone is colored red? and when i try the dragged zone is colored green?
What are exactly the steps to do?
You hold down Ctrl+Shift then draw out the rectangle which initially is green. While the rectangle is still drawn out and keeping left mouse button down, you can let go of Ctrl+Shift and hold down Spacebar to move your selection or Alt key to turn it red for subtracting. Whichever stroke you use for the selection brush will work in the same manner.
Hey thanks!!!