The darkest car I have ever had was a very dark blue metallic, which looked dirty the day after it was cleaned, I will never, ever again buy anything other than silver.
I love silver, all dirt dries to a silver colour and my jazz never really needs cleaning on the outside - only once yearly wash and wax - I do like to keep windows clean because I hate driving with dirty windows.
My neighbours on either side, one has a black BMW coupe, the other a funny dark purple coloured ford focus RS - both are always cleaning their cars, the RS owner never hardly uses his car but it gets cleaned EVERY day (in the garage if its raining - no joke) the black beemer gets used regularly but at least an hour a day spent cleaning it - get a life you two.