I had a Matrox Millenium card with a breakout box for capturing RCA, S-Video, and Cable TV; I'd watch TV on my Windows 98 SE2 computer, which was the craziest thing back then, but I always felt like the green-screen like effect was some kind of mysterious bug that I'd better not mess with, or video capture would break. Windows 98 was barely working on a good day, so it felt like the computer was in the process of failing in a graceful and useful way, so I'd better not push my luck.
Every so often you could get a glimpse of the man behind the curtain, by dragging the window quickly or the drivers stuttering, which would momentarily reveal the green color (or whatever color it was) before the video card resumed doing its thing. Switching between full screen and windowed mode probably also revealed the magic, or starting a game that attempted to grab the video hardware context. And of course sometimes other graphical content would have the exact right shade of color, and have video-displaying pixels.