The transmission fluid starts off as a bright red, transparent fluid. Over time, that fresh batch of transmission fluid will turn darker and more opaque. Slightly older (but still serviceable) transmission fluid appears as a reddish-brown or light-brown liquid and is more opaque than fresh fluids. Once it fully turns into dark brown or near black, then you'll need to urgently change your car's transmission fluid. The primary reason why they get darker is due to a chemical breakdown after constant exposure to immense friction, heat, and pressure within the transmission. Moreover, it also picks up dirt, debris, and other contaminants. Pinkish transmission fluid is much worse, though. This is a sign that coolant or water has mixed in with the transmission; a symptom of catastrophic gearbox failure.