I will attempt to show how I match colors when mixing different cars together.
Step 1.
The 1st step in making a believable image is choosing the right source images. The most important consideration is perspective. Size and colors can all be manipulated but perspective usually can not. I am using the 2 sources I used in the recent Chrysler contest.
Notice how the perspective matches. You don't want to wind up with a car that looks like a dog's rear end or a car that has just run into a brick wall at 40°. Perspective is the key here. Open both source images in Photoshop and compare them side by side. If they look about the same continue to the next step.
Step 2.
Select the front end (or what you are adding to the chop) using the Polygonal lasso tool. After selecting, ctrl+C will copy the selection to the clip board.

You can now close that image and go to the image you are working on.
Step 3.
Paste (ctrl+V) the clipboard contents in the source image. Set the opacity to around 50% and using the move tool, position the added element where you want as in the next image.

Once you are satisfied with the positioning continue to the next step.
Step 4.
Now we have the new layer positioned, we can add a layer mask and remove all the unwanted elements of the added layer as in the next image.

The perspective matches nicely and blends in naturally so we can now start on the color matching.
Step 5.
Now duplicate the layer and desaturate it (shift+ctrl+U). The reason I do this is I want to leave the chrome lights and grille looking natural. They just don't look chrome when desaturated. So I mask out everything on the colored layer except the lights & grille and continue on to my desaturated layer.
Step 6.
Now I adjust the brightness & contrast to brighten the lower (burnt orange color) part of the new front end as in the next image.

Play with the sliders until you are satisfied with the contrast. Remember you want to end up with a natural looking image. Too washed out or too black will not look natural. This is the key to realism in chopping images.
Step 7.
I always mask as close to the original as possible. Zoom in and use a very small brush. Go around the added element until you can see nothing that doesn't belong. Again, realism is what we are going for.
Once this is done we can continue with color matching. This is really quite simple. Create a new layer and set the style to "Overlay" as in this image.
Step 8.
On to coloring. Using the eye dropper tool, pick a color on the lower door of the original car. Using a big soft brush at 100% paint the lower areas of the front end. Now pick a color from the top of the car and paint the upper areas of the added element. You don't have to be fussy when doing this. It doesn't matter if the colors overlap. We are trying to match the way the light is reflecting off of the car.

When you are satisfied with the results, go on to the finishing step.
Step 9.
The final step is cleaning up any loose ends. Look at your work. Add a layer mask to the color layer and mask around the car. Remember the grille & headlights? You want them to look natural so mask any color out that is covering them. Also go around the perimeter of the car. Chances are pretty good that if you can see any imperfections others can too. Be very hard on yourself. here is the final product.
I hope I didn't leave out too many steps. I am assuming everyone reading this tutorial is somewhat familiar with Photoshop and knows the basics like how to add a layer mask, setting the layer style and adjusting the opacity of a layer. Also how important layer order is and using layer masks in general. I have never had any instruction in the use of photoshop, I am just naturally curious. I have stumbled along to get to where I am today. I have found the best teacher for me is trial and error. Know what look you want to achieve before you start and don't stop until you achieve it. IMHO, a 5 minute chop is just that, a 5 minute chop. It takes time and effort to make believable images. I like people to look at my images and wonder if it is edited. I also hope this tutorial has been helpful.
Yer pal, Keb