Dirty is from components wearing, what the interval, you can increase
it by just using the drain plug , it removes less, but its easier.

Sure, the Aisin A series (copy of BorgWarner designs) were relatively
dirty transmissions. That's why you get a strainer and not a filter,
which will plug in no time.

The discoloration you mentioned: if gray then there is clutch friction
wear. This typically accelerates near the end of the useful life of
the tranny. Or the dark fluid with burnt smell, which means
overheating problem.

There really isn't a whole lot an owner can do. Proper fluid and
level, periodic ATF changes, proerly adjusted linkage. It's not bad of
a transmission for its low cost. Sure better than the new U-series
gear skipping junk.