hey guys.... how you all doing. Here i came with a small tutorial

ok lets start

step:1
Lets open car image and here i opened this image and duplicate it.
step 2:
now just desaturate the image or ctrl+shift+u, now the image look like this. ok the second part over.
step3:
now select the car part where you want to colorise the car,.... here my selection and give radius 1.
step 4:
In this step just press ctrl+J , for duplicate the selection and press ctrl+u a hue/saturation window opened... in this you can play with the different radius setting for different colours. here is my measures :)
step 5:
This is the last step ok the colorizing is over, but we need to normalize the car mirrors. Just select the front and side mirrors and give radius as you like and delete it...

Now its over and the final product .