Here's a pretty rough run down of the processes. I would suggest mostly just playin around with the program A LOT.
Select the bottom edge of the body of the vehicle and the background above it. in other words, once you get to the front bumper just go straight outta the screen and include the surrounding background. repeat with the back bumper (assuming your using a 1/2 shot picture). After making your selection, it be best to copy the selection and paste it as a new layer. (Ctrl+C , then Ctrl+V). Then use the "Move" tool to move the body of the truck down on the rims.
Example: Red area is the selected area
Then here it is after you move it down:
Open a photoshop file with the truck you want the rims on. Open another file with the rims you want. Select the rims with the polygon selection tool and copy and paste the rim over onto the truck picture. resize the rims so that they look somewhat realistic on the truck. you may have to color black over the original trucks rims because usually you can see through the new rims into the old.
Select the painted parts of the truck and deselect the non-painted parts (windows, lights, handles). Then go to Image>>Adjustments>>Hue/Saturation. Move the "Hue" bar till you get the color you are looking for.
If the truck is originally black or white, you might have to go to Image>>Adjustments>>Variations. Then click add "Green" or "Red" etc. After adding color to it, then go to Image>>Adjustments>>Hue/Saturation and move the sliders till you get the look you want.