All cars get dirty equally.
Dirt aside, the real issue is with swirls and light scratches in the paint. All cars have them. Just washing your car, or especially going through the car wash, you are having dirt dragged across the surface which is what makes "cobwebs":
This is your average Joe's clean car:
Here's your average pearl white car, with the same spotlight on it:
The swirls are still there, but harder to see. White is less reflective, so when light reflects off it it doesn't show the marks in the paint as easily.
And here's a red car polished 50/50:
As far as DIRT goes... Well that's just a simple color contrast issue. Dirt isn't black, it's light brown so it shows up easily on a black car, and dirt obviously is dark enough to show up on a white car...
Really I wouldn't worry about dirt, it's the scratches showing up that suck.
