i think different shades of grey would be acceptable. I'm at work right now with no photoshop or any of the picture editing tools, but I think the following will be a lot easier on the eye: Light gray background(for white squares), black for dark. White pieces with black outline - so they show on both light and dark squares. Black pieces with white outline - so they show on both light and dark squares.
FFFFFF = white FFFF00 = yellow FF00FF = magenta FF0000 = red 00FFFF = cyan 0000FF = blue 00FF00 = green 000000 = black