You don't need photoshop to do colour changes and basic cut/paste layer jobs.
I use Microsoft Picture it, for a long time I was using the 95 version doing colour changes in a matter of minutes that a few of my mates that are PS keen said were pretty good. I've now upgraded to Picture It 200* something, and its exactly the same, for only 40 dollars or so its well worth it. For Example
Step 1 Open Picture - Higher the res the better
Step 2 Trace your outline, be sure to remove any things that aren't getting changed
Step Three: Well, thats just the final part of step 2: Removing door handles, aerials etc, glass, wipers if you didn't go around them first go.
Step Four: Change Colour: Use the sliders on the left, along with your Saturation, later change the brightness/contrast to suit.
Done, That was a 5 minute job. Can take more time to do the outlines exact.