Explanation of the “same-color” illusion

The Illusion

same color illusion

The strokes in the "green" arm and in the "blue" arm are the identical color, namely [0, 255, 150] which is a blue-green color.

blue green


This image is composed of just three distinct color values. These are blue-green [0, 255, 150], magenta [255, 0, 255], and orange [255, 150, 0], as shown below from left to right.

three colors

In the spiral, they are arranged into three types of alternation patterns, as shown below.

three colors in alternating patterns

When the blue-green is alternated with magenta on the left, it appears to be light blue. When it is alternated with orange on the right, it appears to be green.


The shift in apparent color is due to the spatial averaging, in the eye, of the adjacent blue-green and magenta on the left, and the adjacent blue-green and orange on the right. This can be well predicted by known models of spatial filtering in the visual system. If we put the alternating pattern above through such a model, we obtain the image below.

alternating patterns filtered

Although these may not seem very different, the difference becomes very apparent when we zoom in. If we zoom in by 800% on the original alternating pattern, we see that the blue green looks the same in both patterns.

alternating patterns zoomed alternating patterns zoomed

If we now zoom in on the pattern as filtered by the model of the human visual system, we see the following.

alternating patterns zoomed alternating patterns zoomed

The blue-green has been changed to light blue by the visual system in the alternating pattern with magenta, and to green in the alternating pattern with orange, thus explaining the illusion.

This is the same principle that allows all the colors you see in the patterns above to be created by small red,green and blue rectangles placed side by side on your screen (look with a magnifying glass). The spatial filtering of the eye converts these patterns of red, green and blue dots to all the colors we see on our monitor or TV.