Maybe this is cheating for today's theme, but I use this technique often to help choose color schemes that I am certain will match my images. How can it not? The colored squares make up the image.
Digital images are made up of pixels; small, square, one color.
When viewed normally, the pixels create a cohesive, whole image; you do not see a bunch of geometric squares.
However, you can take any digital image, including scans of images, and enlarge it in photo editing software until you can see the individual colored squares.
I enlarged this image to 3200% and chose just a small selection of pixels to demonstrate the geometric shape...square.