1a. Create New Document
First thing is first, lets start out by creating a new document. Either click File from the menu, then New; Or use the shortcut CTRL+N.

1b. New Document Settings
In this example we're using the dimensions: 600 width x 450 height.

a. Paste Image
Paste this image into your document by clicking Edit > Paste or using the shortcut CTRL+V.

b. Create New Layer
Create a new layer by clicking the 'Add Layer' button on the layer pallet, or use the shortcut CTRL+SHIFT+N.

c. Set Layer Bleeding
While the new layer is selected on the layer pallet select 'Color' from the 'Layer Bleeding' menu on the Layer Pallet.

a. Select Brush
On the Photoshop Toolbar click the Brush Tool, or press B on your keyboard as a shortcut to this tool.

b. Set Brush Size
After selecting the Brush Tool set the brush size to work with, it's best to use a SOLID brush of size 15+ for this image. To set the brush size on the top toolbar click the arrow next to the currently selected Brush. As an alternative you can also right click inside your document to make this window appear.

a. Paint the Car
Now Select your color & paint. In this example I'm using the yellow tone '#FFD300' (hex value). After your color is selected, using the brush tool, paint over the body of your car. You should instantly see the color conversion. For now, cover the entire body, don't worry about making it flawless. We'll handle the touchups on this layer in the next step.

a. Touch-ups
Time to clean up our paint that ended up overtop of our Rear Lights, Mirror, License Plate, and Ferrari Logo accessories.

Use either the eraser or layer masking on the current layer to touch up these color flaws. I use layer masking (if your not familiar with layer masking check back at my site soon for a tutorial, for now use Eraser).

Also touchup the border the same way as the accessories.

a. Solid Color Final Result
If your goal was to just change the color to another solid color this is where you will stop. However if you wish to continue for a few more steps I will show you how to create a 2 color fade!

a. Make Selection
While on the layer with the new color hold in CTRL and click the layer it's self on the layer pallet. This will form a selection around your color on the document.

B. Create New Layer
While the selection is still on make a new layer by either clicking the button on the layer pallet or by using the shortcut CTRL+SHIFT+N.

a. Gradient Fill
While on your new layer with the selection still on pick the 'Gradient Tool' on the toolbar or use the shortcut G.

Now set your forground & background colors, in this example I'm using #FED300 & #D41818.

Now click on your image near where I have 'Start' labeled. While holding down the mouse button drag upwards towards where I have 'End' labeled and release the mouse button.

a. Final Touches
Now to finish it up set the 'Layer Bleeding' mode to 'Color' for the new gradient layer. (Just like you did in Step 2)

Now hide the original new color layer (the gold layer in my example), by clicking the EYE icon next to the layer on the layer pallet.

If you want to try a different gradient or solid color return to step 7 and continue from there. Skip step 8 if your going to do a solid color rather then gradient.

Hope you enjoyed & learned something from this tutorial!