Red & Green for Central Elements

... but not for background or peripheral elements. The retina is largely insensitive to pure reds and greens outside of its central region. When focusing on the

optical center

of a page, reds and greens provide additional perceptual cues. On the periphery, reds, greens, and yellow mixes tend to wash away. If other design considerations put reds and greens at the periphery, increasing the background contrast or adding blinking will increase visibility. Undifferentiated reds and greens at the periphery can be used to cue "secondary" elements of a presentation.