Eclipse of Titan
See how true cyan and green should look.
Stare at the white dot in the centre of the red circle for at least a minute or two (the longer the better). You may blink, but don't move your head, and never take your eyes off the white dot. As you begin staring, you'll begin to already notice a strange glowing effect as if the surface of Titan is melting from the heat. Don't stop now though! Keep staring away, then finally, after a minute or two.... SLOWLY.... begin to move your head backwards.
Witness the Eclipse of Titan!
The green/blue gradient square to the right is not part of the illusion, but just there to show that the colour witnessed is not contained in the colour palette of your PC/monitor (cover it with something black while doing the illusion for proof of this).
The illusion works because of negative retinal afterimage, and the way our eyes' red colour cones are exhausted by the red/magenta colour of the planet. When we move our head backwards, the planet obviously 'grows', and the resulting bright circular rim is the result of appropriate colour components removed from the blue/green sky background. At the planet's top, red is removed to complement the cyan background, and at the bottom, red and blue (magenta) is removed to complement the green background.
The saturation and intensity of the illusion is probably down to a few factors. First of all, most monitors (even cathode ray tubes) are unable to display a very deep saturation of green or cyan because of colour pollution (red mainly). However, monitors can display red (and magenta light to a degree) relatively well in comparison. Therefore, exhausting our red cones to stop them from firing is possible, and therefore, the complementary colour to red - cyan - is made as vivid as possible (the same process is adapted to magenta and green).
It's possible that this saturation of cyan or green isn't possible to see in real life even with perfect inks or paints. This is because, without the illusion, our eyes always receive a small amount of red light 'pollution', and cyan/green is therefore never usually perceived as strong as it could be.
Finally, the contrast of the bright cyan against the semi-desaturated red helps further amplify the effect of the illusion.
Out of all the shades a monitor (CRT or LCD) can reproduce, cyan happens to be the worst in terms of sheer colour saturation, with green following close behind. The amount of red pollution is clearly evident if you take the shiny side of a CD, turn the light off to make the room dark, and fill the screen in bright cyan or #00ffff. On the CD, you should see a great deal of red, as well as the green and blue. This 'shouldn't' be there, since the only light coming from the monitor should be blue and green! If you were to fill the screen with bright green #00ff00, you'll also see some blue pollution and a massive amount of red pollution.