Trans Fluid: Changed 3Qts. Went from Brown to Red?

I just picked up a 1997 4Runner with 130,000 miles on it. The transmission fluid was a brownish red color, so it probably needs to be changed.

To help out a little bit until I get a chance to change the fluid, I drained 3qts out of the transmission at the pan and then added 3qts new fluid back in. The fluid went bright red on the dipstick and has stayed that way so far after driving 150 miles.

Seeing that there is still 12qts of old fluid still in the system, I was figuring the fluid on the dipstick would have went back to brownish red by now, but it hasn't. Anyone know why the fluid is staying bright red?

 

Here is the reasoning I have which appears to make this whole thing confusing.

1. The trans hold 3 quarts and the torque converter holds 12 quarts.

2. The fluid from the trans and from the torque converter both flow into the trans cooler and are mixed together in the trans cooler.

3. If 2 is true, the fluid in the torque converter must be brown if the fluid in the trans is brown. When 3 quarts are changed in the trans, 3 red quarts mix with 12 brown quarts causing the 15 quarts to be brown. Maybe not as brown as before, but still brown.

4. If 2 is false, the trans is not cooled by the trans cooler, only the torque converter is cooled by the trans cooler.

The fluid in the trans has not gone dark again even after about 500 miles. This indicates 2 is false, but I have always thought the trans cooler cools both the trans and the torque converter.