Holden Barina 2002: Dirty transmission fluid

I recently noticed that the transmission fluid in my 2002 XC series Holden Barina (78,000 km) was not looking clean and was darker than it should be. My mechanic checked and said it was not too bad, but we drained it and added new fluid from Holden even though the handbook says the transmission fluid is 'filled for the life of the car'. A week later the fluid is not looking much better although the transmission is working fine and has never given any problems. Is this normal for the Barina, or am I heading for an expensive problem?