Changing the Color of a Car
This is one of the most common Photochops, and this tutorial will describe my personal method for doing it. There are a lot of ways to do it using lassos, masks, channels, layers, and pens, so feel free to experiment. I'm using Adobe Photoshop CS2 on Vista Ultimate. If your setup differs, things may look a little different. Most of what you see here should be the same for most versions of Photoshop.
This is just about the only "tool" you'll be using.
The pen, my personal favorite for making selections, a pivotal part of any photoshopping work. It allows you to zoom in and out and undo your last few clicks, as well as make smooth arcs. It can be a tad challenging to use when compared to the polygonal lasso, but once you get it down it's a much better tool.
First off, you need to find the picture you're going to start with. I'm going to use this
Porsche 911 GT2. I got the original from seriouswheels.com, they're a great place to get wallpapers or photoshop candidates. That, and netcarshow.com. What one site doesn't have, the other usually does.
Here's my original:
Now, we're going to move into selecting.
Using the pen, pick a good starting point, and start selecting the outermost body panels. Don't worry if something like a headlight or window gets inside your selection, we'll take care of that later.
Part of the benefit of using the pen is you can zoom in tight to get a clean selection, zoom out to make sure you're following the right line, zoom back in, select, and repeat. Use this to your advantage.
Remember, the cleaner your selection, the better the final result will look. This part is going to take a while and is going to be a pain, but stick with it.
You can pan (move the window) when you're zoomed in tight by holding the space bar and clicking and dragging.
Another benefit of the pen is you can curve your selection to the image. This makes curves in the bodywork look much cleaner. If you click and drag, you'll see you're dragging the endpoint to the tangent line of the curve you're creating. This may take some practice to get down, but this is what it'll look like:
Keep going...
And going...
And going...
Until you get back to where you started.
At this point, you should have a funny-looking line surrounding most of the car, pretty much everything but the wheels (and in this case, the windshield).
From here, you need to select the things you
don't want to change the color of that are inside that path you just drew. This is where the pen becomes your friend. As long as these things are selected:

(The default setting, by the way) this is going to be easy. Without doing anything, simply start selecting what you don't want to be colored. Headlights, side windows, badges, all of it. Make sure you get it all, or this is going to be a funny looking car.
Badge on the hood:
Front fascia vent:
Mirror detail:
Once you've got all the details selected, your car should look something like this:
Now comes the fun part. Right-click somewhere within the path and select "Make Selection"
Then, set the feather (how soft or hard the edges are, or fade) to 0.1-0.5. Somewhere in that ballpark, depending on how confident you are in your selection precision.
As soon as you hit "OK" you'll see you've converted your path into an honest-to-God selection.
Now, press Ctrl+J. That will execute the "Layer Via Copy" command, which will make a copy of everything you've selected into another layer. It will look like all your work has disappeared, but you'll see you've got another layer of just the body panels over on the layers palette:
This is where your hard work pays off. I would hit Ctrl+J again to make a copy of that layer. Turn the top one off by clicking the eye next to it in the layers palette. Make sure you've got the visible layer selected, then get to work.
The one thing you need to remember is if you don't like what you've done, simply make another copy of the layer you just turned off and start over. There are a few options from this point. You can press Ctrl+U to open the Hue/Saturation prompt, which is by far the easiest to use. Just check the "Colorize" box and your car will instantly change color.
Move the sliders around until you see if there's something you like. There are a few other tools at your disposal, however.
There's the Color Balance prompt that you can get by pressing Ctrl+B:
The Level prompt, from Ctrl+L:
And the Curves prompt, evoked by pressing Ctrl+M:

(This one's great for making metallic or candy sheens.)
You also have the option of making more than one layer, and using a blend mode on the top one, as well as adjusting the opacity. Both of these are on the layers palette:
Combine all of these methods, and you can come up with some pretty wicked colors:
Once you decide on a color, you need to make sure
everything visible is appropriately colored. On this GT2, there's a slight problem. The spoiler is visible through the glass, and our coloring method missed that. However, simply including those parts in the original selection would have yielded some strange and unrealistic results.
However, it's a simple fix. Using the pen and the same method you used to select the body panels, select those spoiler pieces and make them a new layer:
Make sure before you hit Ctrl+J you have the bottom layer selected, the one that actually has the pieces you're trying to copy.
From there, use your friendly Hue/Saturation prompt to make the pieces a believable color:
From there, look over your picture one last time to make sure you didn't miss anything, and you'll have the finished result:
Happy chopping.