To change the eye color naturally, you need to wear clothes that accentuate the color and try different make ups to accentuate pigments in your eyes. The change of the color will not be drastic but it will be recognized with hidden pigments if you practice such things. See http://www.ehow.com/how_5217174_change-eye-color-naturally.html#page=3 for reference.
You can't, it's kind of similar to asking "how do shrink my feet naturally". Naturally pretty much means the body doing everything without any assistance and in it's own time. You can get coloured contacts but they're quite expensive if you want a pair that won't permanently ruin your eyes.