The answer is found in the structure of the feather, or the way the feather is designed.  When light hits the blue feather (remember that white light has all of the colors of the rainbow) the feather is designed to reflect back only the blue light to our eyes.  This can be shown in a simple experiment.  If you take a blue feather and shine a flashlight on it from the top, you will see the bright blue.  However, if you shine the light from underneath the feather, the blue color disappears.  If you repeat this experiment with a red feather, the feather will appear red no matter which direction the light passes through the feather.  Look at the photos taken below of a Blue Jay feather and a Red-lored Amazon Parrot.

Blue Jay feather with light shining from above	Blue Jay feather with light shining from underneath.  Notice how all of the blue has disappeared.
Red-lored Amazon feather with light shining from above.	Red-lored Amazon feather with light shining from below.  Notice how the feather looks red no matter which way the light is shining.

So how are green feathers made?  Touracos are the only bird that can produce green pigments.  All other green birds use a combination of pigments and reflecting blue wave lengths.  Can you figure out what color pigment is needed to make green when mixed with the blue wave lengths?  If you guessed yellow, you are correct.  The wing at the top of this page has yellow pigments, but also reflects blue light.  This makes our eye see the color green.