Hi,
You might try painting them instead of replacing them. If you replace only one, you may notice a difference between the two. Replacing both is gonna be a little spendy.
There are now special paints used to paint this type of plastic (well there have been for awhile, but not available to the Public). Krylon's Fusion Paint Brand is made for this purpose and doesn't chip, fade or crack, and if you do them both, they'll match perfectly. See it here:
http://www.krylon.com/main/product_template.cfm?levelid=5&sub_levelid=10&productid=1751&content=product_details Hope this helps...
Happy Motoring!... Jim'99