My cart will be 8 years old in August and I don't regret the black roof at all. In fact, I don't even know it's black. As stated earlier, the biggest difference is in the seat color. Mine are black and there are days I have to lay a towel on the seat, especially if you're wearing shorts. Roof black is OK, seats black maybe pick a lighter color if you're concerned.
A car whether it gets hotter quicker or cools quicker is a sealed environment, that's apples and oranges when it comes to carts. If you're concerned that much, look in the morning Sun. I mentioned the air conditioned Atomic Kool Kartz, they have a color ad today advertising prices starting at $8300.