Ah, the age old problem: You are looking for a car, but you want it to look good with the minimal amount of effort. I have seen and experienced black car ownership and it can reduce a normal person to a neurotic mess. There are actually 2 ways to go about maintaining the look of your car, and when you use them together you can be assured that your vehicle with always look it’s best.

1. Color. White is hands down the cleanest color choice. There is a reason why a vast majority of the fleet vehicles in service today are white: It is low maintenance. A white car hides dirt, grime, water spots and the like better than any other color with the exception of camouflage. 
Typically the darker the hue of paint, the harder it is to keep clean. The scale rises exponentially between gray and black simply because black acts very much like a mirror. It reflects and magnifies everything including smudges, over spray, fingerprints etc. 
If you are set on a gray color (including the popular silver paints), try to find a lighter gray as it will be less maintenance than a dark gray. Nonmetallic cars are easier to keep clean, especially in dusty conditions as they do not sparkle. A metallic car must be kept very clean to get full benefit from the metal flakes in the paint.

2. Upkeep. Every six months have the exterior of your car detailed. You will be surprised at how much less effort you will have to put into your car’s appearance when you have a pro apply a high quality wax and buff it to a shine. Even if it rains it is not uncommon to not have any water spots on a black car provided it was properly cleaned, waxed and polished prior to the rains. Detailing is expensive but worth the expense if you can afford it.

There are a few products available that will help you keep your car looking factory fresh. 3M makes a clear adhesive coating that is applied over clean paint to guard against rock chips and the like. It must be applied by a professional but it is a lot cheaper to replace than getting the car repainted. American Racing offers wheels that have Teflon baked onto the surface of them which almost eliminates brake dust (which is why the front wheels on some cars are much darker than the rear). Meguiars makes a clear plastic polish that helps take small scratches and light oxidation out of clear plastic (i.e. headlamp lenses)

Naturally, all of these products and services cost money and/or time so it is up to you to decide how clean you want your car to be, once you decide on a color, that is.