Why you should not use red text on blue backgrounds.

First reason: it looks HORRIBLE! (see this example)

There is an optical principle at work, based on the fact that light is composed of different wavelengths.

Being of longer wavelength, blue has a greater refractive angle than red light in the same medium - in other words, behind a focussing lens (e.g. spectacles, or your eye's cornea) blue will focus before red.

In the eye, in order to focus on both blue and red light, your specs should focus red just behind the back (or focal plane) of the eye. To see red clearly you increase the power of the eye (accommodate) until red focuses on the retina.

Unfortunately, the eye has no way in which to decrease in power - focussing the blue on the retina has no benefit.

So by placing blue and red together on a page is like flicking the accommodative switch on and off - it will make your brain ache.