my A4 was Audi 'Dolphin Grey' which was fairly dark metalic.
In winter it did tend to 'hide' general dirtyness quite well, and to be honest, it was really just a workhorse for me, so I wasn't too worried about cleaning it
The TT was a gorgious metalic blue - not great at hiing dirt, but stunning when clean
And the Civ - looks dirty 5 mins after I've finished cleaning it
But it is SO worth it for those 5 mins!