Since there are no blue cones in the center of the retina, these are difficult to see. But blue makes an excellent background color, e.g. for a computer display it tends to blur the raster lines. Avoid adjacent colors that differ only in the amount of blue. Since blue does not contibute to brightness, this creates fuzzy edges. Older operators need higher brightness levels to distinguish colors.
Colors change in appearance as the ambient light level changes.
The magnitude of a detectable change in color varies across the spectrum. It is difficult to focus upon edges created by color alone. Avoid red and green in the periphery of large displays. Opponent colors go well together. For color-deficient observers, avoid single color distinctions.