Glossary » chromostereopsis
a phenomenon of visual perception: different wavelengths of light focus at slightly different depths in the eye. Thus, it is difficult to focus on an image that combines two colors because each color is fuzzy when the other color is in focus. This is especially a problem for images with red and blue.
This problem can be avoided by creating an image without both colors side-by-side, by using black or white boundaries, and by increasing the contrast (difference in brightness) between the two colors.