The "after image" effect is a natural phenomenon of the physical eye.

In effect, the naked eye is quickly fatigued by areas of rich color and what happens is the neighboring colors that are more neutral begin to look like visual complements. In other words, they look the opposite of what they really are. Try this.

In the diagram below, stare at the black dot in the middle of the colored squares for 5 or 10 seconds, then look quickly over to the black dot on the right.

You will see the after image of visual complements in the neutral white space. Yellow moved to the bottom, blue to the top, red to the right side, and greenish-blue to the left side.

OK, you might say, I see what you mean... I think. But, how's that relate to real life and painting? Well, take a close look at the following photo. Make a mental list of the main colors you think are in this sky.

Guess what? The photo above has no blue or cyan.  None!

One reason why the sky looks bluish in appearance in some areas is caused by the effect called "after image ". The longer you look at the photo, the effect of stronger reds and oranges just strengthens the "bluish" complementary effect in the more neutral areas.

Another force at play is simultaneous contrast.

Both of these effects work together to make the sky look blue. Of course it doesn't help either that our life experience also tells us that the sky behind the clouds is blue... right. But that's another topic.

So, if those parts are not blue, what colors are they? The hues are really a range of dull oranges and very dull reds.

What does this mean?

It means that the inner workings of our naked eye fools us into seeing colors that are not really there and not seeing colors that are really there.

The old masters were aware of this and often used it to advantage. They were able to use minimal colors to create the effect of more color than what was actually there.