Well out of my friends here there is: a blonde extrovert, a redhead extrovert and a brunette introvert - although come to think of it they might all be brunettes without the help of hair dye
My hair is brown too although a bit blonder than natural at the moment!!! As for intro/extrovert I have no idea it kind of depends on the day!
I don't think you can generalise on personality or hair colour
From extensive mirror-looking, I can tell you that that theory is crap
Mind you three students/postdocs in my group have green eyes; so much for them being rare. I'm brown haired, one's blonde and one's a redhead. We're very representative.
Ahhh...but PhD students are special, should PhD students match the percentages of the general population, or should there be skews in data and percentages?