Color of tranny fluid when changed

I changed my tranny fluid on my 06 2.3 auto a few months back @ about 20k. Replaced it with Mobil 1. My fluid was a dark brown to almost black color it,did not smell burnt though. I have not had any problems with my transmission. Was this a problem, or normal for the Mazda Fluid? The Mobil 1 has remained a nice bright red since the change(been 5k). For those of you that have done your own change, what has been your experience? If you could, post mileage at change.
Thanks..