I think you mean the screen color picker. You start dragging the mouse from that box to the color you want. You can view a zoomed preview in that box.
If you mean the movie mode, it detects the most common color, not the accents. The problem is that the bulb doesn't support black or darker colors, so sometimes the bulb shows white or any other random color. Right now, if the screen is completely black, I've set it to turn the brightness to the lowest level. I'll try to fix the random colors, but I don't think there's much I can do.