Having done this many many times for users of PF I thought i'd make a quick tutorial to allow people to experiment themselves with colour changes.
This is by no means the complete way to do it, there are several ways depending on the original bodywork colour and the colour to be changed to. for example changing black to anything else is difficult to get right and as such uses different methods.

I have now added a second tutorial showing another way to achieve this colour change.

This tutorial will however work for most cars. It does not show every single step but most of them. If anybody wishes to know more about any part of it, feel free to PM or Email.

Method 1

Step 1.

Start with a picture.


step 2
Right click on that layer and select duplicate layer (or press control and J)


step 3
Then select polygonal lasso and do a rough outline of the car only, but dont get too close to it. This will save some later in a later step.


step 4
Once you have done this goto, Image/adjustments/hue & saturation.
Click on Colourize. you can then alter that layer's colour using the Hue and saturation sliders.
When you have decided on a colour select OK.


step 5
This is what you should be left with


step 6
Now using the eraser, and using the zoom function remove all the outer section of colour that have gone beyond the bodywork.
Also remember to do the windows, lights, petrol caps,wing mirrors and trim etc etc.

step 7
you should then end up with this.


step 8
Then by returning to the hue & saturation option and selecting colourize again, you can now change the car neatly.





You may notice that the blue of the handles, petrol cap window seal etc comes through, this is not a problem. You can either chose not to erase it when carrying out step 6, or otherwise you can creat another layer and reduce saturation to 0, this will make them look dark grey.

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Method 2

Open the picture you wish to use.





Duplicate the layer to work on.






Goto Image/adjustments/replace colour







Move your mouse to the +icon (add to sample)







Click on it








Now you should get a wand, drag this over the bodywork of the car only on the coloured section in this case RED, avoid any of the other colours nearby such as black doorhandles, door creases etc.
Then when you've finished (dont worry about doing too much as you can add some more if you haven't done enough) goto the HUE slider and slide it either way and you will see all the red you highlighted change colour.
If some RED parts have not changed just go back over them with the wand and you should see them change again.






Here you can see how the colour change function alos changes the colours of all things RED in and around the image, in this case trees branched etc with red in them have also changed colour.







You can then just delete them either by highlighting them and selecting delete of using the eraser.








Once done you should have something like this






Note:
You may sometimes need to adjust the Fuzziness to get a clear colour change.

You are not limited to the HUE slider alone, you can also DESATURATE to create Greys and also use the Brightness slider to get anywhere from Black upto White.

I hope you find this helpful.