A color has two components: hue (e.g., green, red) and luminance (i.e., brightness). Equiluminant colors differ in hue, but not in brightness. When foreground and background colors are equiluminant, the content of the image is hard to perceive:
Why is that so? After all, the two colors used to paint the sign are clearly very distinct:
It turns out that human brain has two separate pathways in the visual system. One is colorblind and processes just the luminance information. The other one can process hue information. The colorblind pathway is high-resolution and fast. The color-sensitive pathway is low resolution and slow. Thus, it takes us longer and requires more effort to process visual information that is only encoded using hue, but not luminance.
So equiluminant color schemes are bad, right? Not necessarily. It would be a terrible idea to use equiluminant color schemes for road signs or presentation slides or in other situations where visual information needs to be easy to use. For example, in one of my classes, a student group gave a presentation using a color scheme that looked roughly like this:
Very elegant, but it was nearly impossible to follow the content of the slides without getting a headache.
In other situations, making visual information a little hard to process may actually cause people to comprehend the information better in the first place and remember it better later. As pointed out by Hullman, Adar and Shah (2011, see below), making visual information hard to perceive essentially tricks people into exerting a little more effort processing that information. That, in turn, might improve comprehension and recall. According to Prof. Livingstone (see the video and the book below), this trick is often used by advertisers. This is a mean trick in the sense that it gets people to exert more effort interacting with an image than they might have wanted to, but it may help you get some extra attention from a reluctant viewer.
Prof. Margaret Livingstone at Harvard Medical School has a fantastic lecture (see the video below) that explains the neurobiology of visual perception--including the two visual processing pathways--in a way that is both informative and accessible to non-experts. She also has a very good book, Vision and Art: The Biology of Seeing, that covers the same concepts in more depth.
Additional academic publications
Hullman, J., Adar, E., & Shah, P. (2011). Benefitting InfoVis with visual difficulties. IEEE Transactions on Visualization and Computer Graphics, 17(12), 2213–2222. http://doi.org/10.1109/TVCG.2011.175