I'd add one suggestion to Steve Kinney's excellent layer mask tutorial:
Once you have the car layer masked off, and you've dropped or
painted a new color onto the masked layer, try using layer modes
(the option menu in the Layers dialog) on the color layer. When you
drop a solid color, it won't look right at first, because it won't
have any shading or reflections. Layer modes let you combine a
solid color (or gradients, or stripes or other patterns) with the
lighting from the original photo so it looks a lot more real.
Color mode is the most obvious, but step through each layer mode in
turn (you can use down-arrow once you've clicked on the Mode menu)
to compare the effects. Some of the modes, like Difference or
Overlay, can give you fancy pearl effects that look great on cars.
When I give GIMP talks I almost always demonstrate car colorizing
(and I have a section about it in chapter 10 of _Beginning_GIMP_)
because the results are so impressive.
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