Replacing one colour with another in an NSImage

I have an NSImage and want to replace one color in it with another (e.g. replace all the blue with a green colour开发者_开发百科) Is there an easy way to do this and if not, how could I get this functionality?

This is a job for core image. Or, you can use use Quartz Composer to figure out your image filter, and embed a QCView in your app.