We're dealing with Light and not Paint so:
What color does green and red make?
In: Colors
The area of your eye that detects color is called the "cone". Your cones are specialized to send the messages for "red", "blue" and "yellow" to your brain. The absence of red is green, just like the absence of white is black. Thus, under normal conditions, there's no way to combine red and green. However, there are very specific conditions that cause your retinas to receive both a "red" and a "green" signal and send both signals to your brain, resulting in "reddish green". Doing so is difficult, one has to flash red and green in quick succession before the person trying to see reddish green while trying to keep that person's head very still so not to disrupt the illusion.
When you blend red light and green light of equal luminance, they cancel out. Since most forms of light contain RGY, most people see yellow rather than white when this "cancel" happens. The reason why green and red paint turns brown is because you've overloaded the paint with pigments, it's absorbed so much over every color that it can't display yellow or white, but your eye is canceling out either the red or the green (as well as the blue or the yellow) because you can't normally see both at once.
Read more: http://wiki.answers.com/Q/What_color_does_green_and_red_make#ixzz1A6Kz7pWu
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If this answer I found is not correct, I apologize. The next time I see a yellow tick on my compass, I'm going to head straight for it.