One reason for the strain (Score 1)

The human eye not only needs to adjust focus based on distance, but also on colour as different wavelengths refract at different angles (a phenomena exploited in certain opticians' tests) It follows that having blue text next to red will strain the eye, because the eye will need to change the focal distance when switching between the two. I just viewed a single blue line of text, amongst other red lines. I did indeed notice that the blue appeared slightly blurry. Perhaps a single colour (green?) on a black background would provide the least focal problems. To compensate for other strains, the background could be lightened slightly?