This time last December, I did a little test of my twice drain and filled transmission fluid. Purchasing ten pair of baby cotton white socks, washing in bleach and detergent, I took a drip on one sock"s toe of the fliud from the dip stick. Then took a drip from a little new fluid I had in a bottle of Type VI and placed it on the heal. Then took a felt pen and dated the sock and placed the sock in a zip lock freezer bag.
The last four oil changes I dated and used a sock, drip from the transmission and drip from the bottle. Laying out the socks today, I see the fluid getting darker but still a red color.
I dated the year on the bag and will compare again next year.