Øyvind Kolås is an artist and developer and a prominent member of the GIMP open-source image editor project. He came up with this illusion of adding colour to greyscale images via superimposing a coloured grid over them as a result of working on improvements in the saturation operation of GIMP.
“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,” he writes. “In this illusion the reconstruction is happening in our eyes/mind - but it uses the same principle that Chroma Subsampling does, that luminance is a lot more important than the chroma for our visual perception.”
The original post is an interesting read as he goes on to show different examples of using different patterns to get the same effect, including dots, lines, and text.
And, given his position in GIMP development, if you want to experiment with the effect yourself it will be available in stock GIMP versions with the next GIMP-2.10 release as well as in any upcoming development snapshot for 3.0.
Here it is below working on a greyscale version of the Blender Open Movie ‘Spring’.