Why? The brain’s visual cortex has far more neurons devoted to finding borders than colors. That’s because borders are much more useful in identifying shapes. Once those neurons identify the dark blue border pictured here, they search for symmetry. Without that inner blue line, the color inside the dark blue border is the same as the color outside. But add a light blue line inside, and your brain grapples with the change. It guesses at the interior color based on the color closest to it—in this case, by incorrectly assuming the entire inside is blue too.