Hair color in anime is often used to depict traits about a character. This is by no means a complete list, nor does it mean every character to have these hair colors acts like this. I'll provide a few examples though.
Pink haired characters are often spunky, curious and bubbly (Chibi-Usa/ Rini from Sailor Moon, Sakura from Naruto).
Blue hair often signifies a calm, sometimes shy demeanor (Hanon from Mermaid Melody), but just as often signifies a character who can be over the top and hyper (Konata from Lucky Star).
Black hair, especially with spikes is a common trait of shonen characters, especially those who crave action and excitement (Ash Ketchum, Goku from Dragonball).
Red hair (as in the actual color red, not orange) is often found on boistrous, loud characters, or those with firey tempers (Hikaru from Magic Knight Rayearth, Jessie)
White, silver, or grey hair on characters who are not elderly often implies an air of mystery (Rei from Evangaleon), but can also signify a villain (Sephiroth, even if he's not from an anime, Sesshomaru from InuYasha; I'm leaving his brother out of it because he has more dog-like features, justifying the white hair in his heroic status).
Bald characters are almost always villains (Lordgenome from Gurren Lagen, Vargus from Negima), unless they are Chinese-styled monks (Krillin from Dragonball).