I'm using a 64bit version of Photoshop CS5 on a 64bit windows.
I'm using the "Quick Selection tool" to select only the painted parts of the car. Takes a while to +/- all the small features, zoom in heaps to omitt (-) lights, badges, window frames, windows etc.
Selecting the painted parts of the car only makes it easier to play with colour changes.
If I'm using colours from Photoshops "colour pallet" or "eyedrop tool" I use the "Colour Replacement tool" to change the colour. Right click on the "Brush tool" and change it to "Colour Replacement tool".
With only the painted parts of the car "selected" you can wipe "colour change"over the whole pic and only the selected components will change.
The Vectra was done with Photoshops colour pallet as was ZFL45H1s' pink FTO.
When I want to use a colour from an image, I have the car pic I'm changing and the donor car pic I want the colour from both open seperatly in Photoshop.
Photoshop will use all colours present in the donor pic, so I crop out a piece of the donor car and Photoshop has to use the small sample I have chosen.
If the donor colour still isn't quite right I will try a crop from a different part of the donor car, as reflections within the donor paint make a difference on the outcome.
I then use "Image/Adjustments/Match Colour/Source" and select the donor colour/car/pic from the drop down menu. Then I play with "Luminance" and "Colour Intensity" settings to my own liking or to closely try and match donor colour.
ZFL45H1s' "purple" and bjks' "Mazda Grey" FTO's where done this way.
I have found it also depends on the colour/reflections/light source within the pic you want to change.
Light colours will go over light colours (ZFL45H1 pink FTO), dark colours will go over dark colours(ZFL45H1 purple FTO), metalic colours will look best on an already metalic car(Vectra).
Pink Vectra and pink FTO were done with the same pallet colour, it was the original car colour that made them look different.
With ZFL45H1s' "Purple" FTO, I had to select all the painted parts and "Invert" the white FTO paint to make it dark to be able to apply a dark colour "purple"
After all this, if required, I play with "Image" "Adjustments" tools to get the final pic.
When I have the desired outcome, I "Save as" + "rename the pic" and then I can "undo/step backward" changes, back until only the "Quick Selection" is still in place and go again with a different colour. It just takes too long to keep having to do "Quick Selection" again.
Oh and "Pure awesomeness"
I hope I have given enough info.