Why is the transmission fluid dark?

Heat is the main reason for darkening. If it is truly dark, perform a drain and fill, to keep it nice and red.

I would be looking at adding a transmission cooler or changing the fluid every 10k miles.

IF YOUR TRANSMISSION IS HEATING UP. ALSO CHECK IF THE ENGINE BAY IS GETTING ENOUGH AIR TO KEPT THE ENGINE AND TRANS COOL. THE MUSTANG HAS THAT PROBLEM.

Are you pulling a trailer or camper? If so the vehicle may be overloaded! Hope that helps! Jim