All right, I've reviewed the Green sky bug a little, and I tnink I knw what happens...
It looks like the sky is roughly cubical in shape. The green appears when Neil Cubestrong is in the air, and the camera is set up in such a way so that you can't see the ground. Neil will freeze in the air while the mouse button is held down (or possibly he's back on the ground, and you're looking through the bottom of the floor), and while you can still rotate in a circle, the green appears when Neil overlaps corners. If the camera angle is tilted enough so that the ground would become visible, or you let go of the mouse, the angle jolts back to normal, forward vision (and the words "COLLIDED BOTTOM" appear).
Is that clear at all?