Can anyone give me some pointers on changing objects in photoshop?

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Hello, Is there anyone out there who can give me some pointers on changing colors of objects such as cars, eyes, and backgrounds in photoshop? I am a freshman Graphic Arts student, and while I have several projects my instructor gives us permission to use photoshop on, we haven't really learned much about photoshop yet...I've used the free program "Gimp" which seems to work alot like photoshop, but since I don't know much about photoshop I don't know for sure if Gimp is just as good...Can someone answer this question too? Thanks