Has anyone got some sort of application which you can use to play around with different colours, on a virtual car?

I believe there is a colour change bit on the westfield site.

http://www.westfield-world.com/flash_changer.html

You could try the liveforspeed (the best racing simulator) skinviewer. You can download it for free at www.liveforspeed.net under additional downloads -> CMX viewer S2S. Making your on paint scheme is very easy, it just needs a 1024x1024 .jpg
You can use photoshop templates to make it even more easy.

example: ftp://elitesqd.com/_users/lococost/lfs.jpg