Viral trends, just like the biological viruses they are named after, are forgotten and vanish as quickly as they appeared. So just in case you didn't pay attention to social media trends back in late July: an interesting visual illusion made its viral round on social media. The attraction point was a purportedly monochromatic image, yet the image clearly appeared colored in the eyes of the viewer.
The image itself is indeed using nothing but gray tones. However, it is covered with a carefully layered grid made of thin colored lines. Like a nearly transparent spider's web, the grid merges into the black and white background and is indeed barely noticeable. However, the presence of the colors in the grid lines forces the brain the compensate for the contrast and automatically make up for the missing colors in the image's own elements.
The "color assimilation grid illusion" is an interesting trick, though I'm not sure at this point how you can use it during a party. It may have some interesting applications in the future -- such as providing a way to digitally colorize archived visual documents, and do so without altering the original material. There are several drawbacks and limitations to this technique, the most obvious being that the resulting colors appear significantly washed up. The presence of the distorted grid can also be distracting. Finally, the displayed image is not technically devolved of colors: the grid provides Which means that technically, the premises are misleading, as the displayed image (the combination of a black & white photo and a grid of colored lines) uses hues.
It's interesting to note that the magic even works on moving images: as the clip included in the referenced article (and embedded above) proves, monochrome videos rigged with the same grid overlay will appear in color. The author of this technique, Øyvind Kolås, a GIMP developer, plans to integrate the technology in a future iteration of the graphic editor.