Color replacement

I have been trying to figure out how can i change the background color of a photo or better yet.Someones shirt in a photo from green to blue?I have seen it done but i cannot figure it out.Any help would be greatly appreciative!
To elaborate, since i do not think i explained this well enough.I want to be able to change the background horizon of a photo to yellow if the color is green.I am using Aperture on one iMac and Aperture 2 on the mba.So if im not making any sense please let me know.
Regards

nothing on this subject?

ibookmobile007 said:

nothing on this subject?

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Do you have iwork 08?

You can't do what you want to do in Aperture as this requires creating layer masks which Aperture does not support. If you have Photoshop you can use the replace colour filter or make a selection (mask) and use the Hue/Saturation slider to change the colour.