Simultaneous contrast identified by Michel Eugène Chevreul refers to the manner in which the colors of two different objects affect each other. The effect is more noticeable when shared between objects of complementary color.
In the image here, the two inner rectangles are exactly the same shade of grey, but the upper one appears to be a lighter grey than the lower one due to the background provided by the outer rectangles.

So here the upper forklift looks worse because someone stuck a pud on it, while in reality they may be roughly equivalent. Not fair is all I'm saying.