I was always fascinated with blue-dot tail lights when I was a kid. Personally think that they are overused on old cars today, but that's just my opinion.
Anyway, usabilityfirst.com explains the effect as
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 image shows why we don't have blue tail lights with red dots!
