Tutorial: Colouring a white car
Strange it may seem, but without the correct steps, it is extremely hard to get any type of colour onto a white car. Here is the best method I can possibly think of.
 Colouring white car
Difficulty - ***
Guess this isn't necessarily a white car, but it is a light shade of grey nevertheless. Basically the idea behind this tutorial is to darken the paint for some colour to "bite" onto.... If you catch me!  
Right.. This is how we do it. Listen up! Go over to the box in the bottom right corner, right click the layer and go to "Duplicate Layer".  

We're getting there now.. Desaturate now, by hitting CTRL+D.. Then go to Image>Adjust>Brightness/Contrast. Then kinda change the settings to to the same as on the left.. Just make the paintwork look like in the piccie on the left.  

Right, head over to Image>Adjust>Variations, and whack some colour onto it.. What ever colour you like, you add as much... green for example as you like by clicking the green box many times. IT should now look like the left pic.  

Use the Lasso tool (or tap the L key) and begin selecting and deleting (del) the areas you don't want to be green.. EG the windows, and the background.. You catch my drift?  

Carry on cutting out the pieces and once you've done all that.. its complete :)

Don't forget to flatten layers (Ctrl+Shift+E).