Automatic transmission fluid darkens with use and this is normal.  Transmissions also generate a small amount of debris as bushings and gears wear with use. Debris falls to the bottom of the transmission pan and is sucked into the automatic transmission filter.  When a transmission service is done, cutting the old filter open provides insight into the condition of the transmission.

Judging the level of metal that shows a problem is difficult without experience.  No metal is best, but a small amount of very fine particles may be normal in some transmissions.  Larger pieces as in the illustration above are always a problem.