That's correct. Although her style as a whole was more consistent in the previous games before Conviction. In Chaos Theory and in Essentials she looks dirty blond... kind of. It was also easier to make her hair brown in in the first SC. With the graphics of the time, dirty blond didn't look so good.
Yes graphics enabled them to advance the in-game and cut scene models for principal characters. Sam's model has changed over the course of the games - as in DA he was bald for much of the game as per the prison hair cut.