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Did you see the correctly colored Red White and Blue flag? If not, go back and stare at the flag a little longer before pushing the next button. The reason you see the correct red and blue colors is that those are the colors which oppose green and yellow, respectively. When you hold your gaze steadily on a bright color, your eyes tend to inhibit stimulus from that. When your vision is suddently replaced with white, the inhibited colors tend to be temporarily removed, leaving opposing color.
You can get a similar effect on a bright day by closing your eyes and pointing your head up toward the sky. Your eyelids filter most of the bright light, but a strong red tinge will still reach your retina. When you finally open your eyes, you will see that the colors around you appear wrong.