Overfilling a transmission causes the fluid to become aerated and foamy. It results in leaks, buzzing and whining sounds, and erratic shifting. Fluid will seep past the dipstick or bypassing axle 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. Worn and damaged metal parts 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.