Interesting. When I had a TVR it was accepted that cars would have their colour changed at some point in their life. I think this was a combination of the fact that most of them ended up accident damaged at some point, that it was all just fibreglass panels, and that they were pretty much available in any colour under the sun anyway.
Not sure I'd do a colour change on a Z4M, although it's easier if you're going from a colour to a similar colour (e.g. white to white with blue fleck) as you don't have to do all the shutlines, inside of boot, under-bonnet, etc.
It seems that colour changes on MX-5s are relatively common too.