Oh, well, that color-on-a-black-and-white-image actually is a case of adaption. It happens for all senses as well:
Perhaps you've been to a loud room before and then left and found a really quiet room. If you did that you might have noticed how everything you hear sounds kind of dull. Only after a while your full ability to hear is restored. This is equivalent to the afterimage in vision.
Similarly, if you smell something when entering a room, even if the smell persists, eventually you won't smell it anymore. But if you then go to a neutrally smelling place, you'll "smell something" which is actually just the lack of that other smell.
And if you touch something over a longer period of time, you cease to feel it. - For instance, unless they are seriously uncomfortable, you won't constantly consciously feel the clothes you wear. It's the same effect.
This is a result of how neurons work.
Neurons essentially are constantly in a state of imbalance when NOT activated. Electrical charges are separated in them.
If they are activated, those charges suddenly can move freely and there is a "spike" which ripples down through your nervous system.
After that, those neurons are discharged and they need some time to recover again. During this time it's impossible for them to send along another signal.
Now if you have constant exposure to something (like, say, a static black-and-white image while not moving your eyes), some of those neurons are constantly being told to trigger, even when they can't. That means the overall effect of the signal is dulled and diluted.
For the eyes specifically there are some extra subtleties beyond that (and this generally is rather simplified) but really the basic idea is always the same. Some cells are usually in a "ready" state, then they "discharge" and then they have to "recover" during which time their ability to do their task is more limited which, in distribution, causes an apparent weakening of the original signals.
