The title of the thread alone made me smile, but now the posts are seriously making me laugh.
In all honesty, I think unnaturally colored hair is really cool on the right person. I worked at a small cafe in college and often waited on a girl with punk-type style who had all her hair shaved except for a wave in front that was dyed turquoise blue, green, and purple (yes, like a peacock.) I admire people who are willing to be a little different (so long as it's not breaking the law or something) and don't care what other people say or how long they stare. I went through a time of dyeing my hair burgundy red and, at one point, a kind of eggplantish color so I don't see anything wrong with experimenting with your hair color.
As for guys... I've been attracted to all types of guys--loud, quiet, taller, shorter, athletic, brainy, preppy, alternative, etc... I've always had a bit of of a weakness for art boys as well, and they often do unconventional things with their hair. I actually liked it when it was cool for guys to bleach their hair and then grow it out, producing a two-tone effect... on the right guy, it's quite attractive, in my humble opinion.
Rainbow hair would not be a problem for me unless it kept him from something important in life, such as getting a job.
P.S. This thread is making me fear that I'll never be 4 popped collars cool... LOL.