Each color has two components: hue and luminance. Equiluminant colors differ in hue, but not in brightness. However, when foreground and background colors are equiluminant, the content of the image is hard to perceive. Take for example:

Why does this happen? The two colors pink and green are obviously different. This is become the human brain has two separate pathways in the visual system. One system is colorblind and just processes the luminance information. The other system processes hue information. It is important to keep equiluminant color schemes in mind as it would be a bad idea to use equiluminant color schemes for road signs or presentation slides.

Contributed by Janet Ho and Maurice Wilson

… This has implications for memorable design: if we want to create visual designs that are easy and fast to process, equiluminant designs are not a good choice; but on the flip side, if people choose to go through the effort of processing perceptually difficult, equiluminant designs, they are more likely to remember it due to the increased amount of time interacting with it.

Contributed by Luke Xu, Elena Wu-Yan, and Manasi Maheshwari