Curves also works this way with the right and left sliders.
(Just to be clear) Leaving the slider at 0 or 255 shows you what you have, moving them shows you what you will have if you make that adjustment. The color is which channels are at 255 or 0 for each pixel (for the right hand slider, for instance, white for all three channels, yellow for red and green channels, etc.).
Cheers,
-Michael-