the negative of color image doesnt change to B/W. On rollover the B/W is placed in place of that color channel pic. Now since we have been looking at that color channel pic it gets superimposed on the B/W image and we see the colors.
Its the same as that picture of jesus illusion you saw..
Ill explain it scientifically. When you stare at a screen and move your eyes away you see some ghostly images which fade in a few seconds. This is called complementary afterimages. So you actually see the complement of that image.
SO when you stared at that image (which actually was the complement of the original image) of jesus, your eyes got fatigued and when you moved your eyes away and focussed on a wall you saw the original image of jesus due to complementing afterimages. Complement of a complement is the original.
Similarly here you are staring at the complement of the color image. And now when you suddenly see the B/W image when you move your mouse over that picture. You get the complement of the complement of color image, which gets superimposed on the B/W image to show you colors.