This black-and-white photo tricks your brain through a process called Color Assimilation Grid Illusion. By laying thin color grid lines over the top of it, your brain to perceive and fill in the missing colors itself.
There are several ways you can look at this photo; stepping back and glancing at it from afar, or squinting while staring at it can both enhance the colors you see.
The experiment comes courtesy of GIMP’s open-source image editor project artist and developer Øyvind Kolås, and has been picking up traction on the internet this week. The photo itself is one licensed through Creative Commons, originally taken by Chuwa (Francis). It was converted to black-and-white, before being overlaid with red, orange, yellow, blue, and green grid lines.
An over-saturated colored grid overlayed on a grayscale image causes the grayscale cells to be perceived as having color.
Below is another image from Kolås, created using the same process:
This is not the exactly same as the way JPEG compression works, since in JPEG compression the lower resolution color signal is present for in every reconstructed pixel, in this illusion the reconstruction is happening in our eyes/mind. [B]ut it uses the same principle that Chroma Subsampling does, that luminance is a lot more important than the chroma for our visual perception.
Kolås is making the technique available via GIMP as an operation called color-assimilation-grid, available starting with the next GIMP-2.10 release, meaning you can try it out on your own images.