Those three different topics are also in three different subject matters so I decided to break them into 3 separate articles.
And our vehicle will be a Toyota Aygo I've got this from flickr under CC license; we will be using this image for this tutorial.

First thing you should do is to duplicate the image [CTRL J], then using color range pick the white tint and mask it. For more info about color range and masking click here.
Clean it using a hard brush, you should be having the same result.

Then go to your new adjustment layer button and choose Solid color, apply [#3c003a], you can change the color but don't cross the border that I indicated on the figure if you still want your tint to look natural.

After that Change its blending mode to color, then link it to our Toyota aygo with the layer mask, linking can be done by holding alt then click in between them.

That's it, that how you change the tint of your auto, I know advance chopper already knew this method but I have something new to show. We will be making the tint more dynamic.
First set your background and foreground color, what ever you want is ok with me.

Then make a new layer, grab your linear gradient tool [G], and fill it with those settings, how you would do it is all up to you, just don't forget to set the blending mode to overlay.

Repeat the procedure with different color gradient, then on the upper most layer create a duplicate of our original image and apply a curve adjustment this will allow us to add spanking effect to our photo. (Use the S curve) and don't forget to change the blending mode, you can choose overlay or Hard light, but I prefer hard light coz I wanted the shadows to stay intact.

This could be helpfull for those who wanted to repaint their vehicle, using this method you can show them a sample of what exactly you wanted them to do.
These are the final result for this tutorial.
1. Common pinkesh tint using single color fill.

2. Dynamic tint using overlayed gradient.

I would be creating a flash widget that enabling the user to choose from different kind of colors available for purchase soon, it would be about color transformation using actionscript.
discuss this topic to forum
