Why do many characters tend to have crazy hair colors and styles? It's fairly common that anime characters tend to have "wilder" hair than most people we see in real life. This can be seen as far back as Neon Genesis Evangelion (though somewhat mild) from the 90s (though some older anime, like Akira, do not have this characteristic).

The "wilder" hairstyles tend to have interesting colors from throughout the rainbow (pink, blue, green, etc.) that are only seen in real life with dyes or highlights. They also tend to be spikier, another trait not seen in real life (probably because gelling one's hair that way every day is quite tedious).



Why do anime and manga characters tend to have this spiky, colorful hairstyle? What is the cultural origin of such a style? (And did the growing trend of Japanese teenagers spiking their hair in a similar style originate from this?)
Most likely it happens because it looks cool and unique. Here's what tvtropes say about anime hair:


  Usually, the most important characters of the story will have wild
  spikes or a cool-looking hairdo in order to stand out among the crowd.
  It may be one or more different colors that don't appear naturally in
  real humans (blue is a popular choice). Sometimes the hair appears
  semi-transparent, with the character's eyes visible through it,
  although this presumably represents hair fine enough that it isn't
  completely obscuring, rather than anything outré. Anime Hair is very
  common among protagonists of anime/manga for the Shonen (Demographic),
  although the trend seems to be headed to more plausible styles:
  compare Son Goku's hair to Ichigo's. If there's a White-Haired Pretty
  Boy in the cast, there's a good chance that white hair will also be
  Anime Hair.


Now, compare this Vash picture:



with this one:



and ask yourself, which hairstyle is more bad-ass? 

Some of the characters need to stand out among the crowd, and weird hairstyle/hair-color is a very easy way to achieve that (weird clothes are popular too). Also, it's not uncommon for the hairstyle to somehow reflect the character's personality, e.g. spiky hair for characters that need to look wild/cool, adding ahoge for some more naive/funny characters, and so on. 

Hair colors can also be somehow used in the plot, e.g. Ichigo being bullied because of his uncommon hair color. Sometimes, the color is also used to indicate some of the character's personality, here's an interesting link.

Finally, as @Tacroy mentioned in his comment, the same face may be used for different characters, and changing the hair is the easiest way to make them look different.