If your vehicle is showing red or brown fluid leaking under the middle of the car, here are the most likely causes and the parts you'll need to fix each one.
The seal around the transmission fluid pan has degraded, or the drain plug was improperly tightened/reused during a recent fluid change. This is the most common source of red or brown transmission fluid leaks.
The seal at the rear of the transmission where the driveshaft connects has worn out from normal rotation, allowing fluid to seep out and drip down the middle of the vehicle.
Engine oil (which turns brown as it ages) is leaking from the rear of the engine and blowing backward while driving, making it look like a leak under the middle of the car.
On four-wheel or all-wheel drive vehicles, the transfer case sits in the middle of the chassis and uses red ATF or brown gear fluid. Its seals or case halves can dry out and leak over time.
The metal lines carrying hot transmission fluid to the radiator cooler have rusted or chafed through. While they usually leak at the front, fluid can travel along the lines and drip from the middle.
A major internal failure, severe overheating, or impact has blown out the torque converter seal or cracked the transmission case, causing a rapid, massive loss of fluid.