Hair color is another thing people notice. I remember watching Neon Genesis Evangelion once at my home, and my mother walks into the room and sees the character Rei Ayanami - a young girl with short blue hair. I really didn't know at the time. Manga is traditionally drawn in black and white, and so different shades were given to different characters to help differentiate them from each other. In anime they were given different colors for the same reason, however it is not the only reason.
Blue hair can represent a calm personality, fire-red hair can represent a hot temperament.
Blondes often signal trouble or evil. Other facial features can help define a character. An American, for example, can be represented by freckles on the face of the character or an elongated nose. Small nuances like this help define the character and give them individuality from each other. It's expensive, it's not for everyone, and in some places it is very hard to get a hold of.