Overfilling a transmission causes the fluid to become aerated and foamy. Someone may have added fluid to a cold transmission not realizing that fluid expands when heated resulting in an overfilled transmission.
It can be seen seeping past the dipstick or bypassing seals. Transmissions are sealed and do not consume transmission fluid. When the fluid is low, look for a leaking transmission pan gasket, axle, or pump seal.
Pieces of black particles in the fluid indicate slipping friction materials. Wearing metal components result in silvery metal pieces in the fluid. If the fluid’s pink and milky, it’s become mixed with coolant in the transmission fluid cooler located inside the radiator.