If you take a step back or gently blur your vision, you might think this image (below) is a fully colored photograph of some students in a classroom. However, look a little deeper and you’ll realize it’s actually a black-and-white image, only given the impression of full color by a subtle criss-cross of red, blue, yellow, orange, and green lines.
So, why is our brain being so predictably foolish? Created by software developer and digital artist Øyvind Kolå, the image is an optical illusion known as a color assimilation grid illusion.
"An over-saturated coloured grid overlaid on a grayscale image causes the grayscale cells to be perceived as having colour," Kolås explains on his Patreon page.
The image is an interpretation of a chromatic illusion known as color assimilation. Much like other examples of chromatic illusion, such as the Bezold effect or the shadow illusion by Edward H Adelson, these images exploit the way our perception of color is influenced by adjacent and interspersed colors. The effect is called "color contrast" when the perceived color shifts away from the surroundings, but is called "color assimilation" when it shifts towards that of the surroundings.
Precisely why our visual system does this remains a bit of a mystery; however, it’s generally understood that the brain and eyes attempt to ensure our perception of color remains relatively constant regardless of the illumination conditions surrounding an object.
If this wasn't the case and we just perceived colors at face value with no wider context, an object would appear dramatically different in various environments and, chances are, end up confusing you. Just think how different a poisonous berry might look under the evening Sun compared to a rainy day in a shadow. Our visual system tries to account for this, allowing us to recognize the object clearly even if the context has changed.
Most of the time, we have no conscious idea that this is happening and the effect works to our benefit. However, with chromatic illusions like this, the effect is played with and plainly revealed.
Kolås also created a short video (below) using the color assimilation grid illusion on a moving image, which demonstrates the effect even more brilliantly.
Kolås’ work is distributed under a Creative Commons license, so you are free to use, edit, and play around with the image as much as you want. Just make sure you credit Øyvind Kolås and you check out his Patreon page if you want to support their work.