Both the VN and the anime stress that Michiru is a fake blonde.
Makina and Yumiko have natural hair colors...
But what about Amane and Sachi? I don't think there are too many natural redheads in the native japanese population of the Kyoto region (where Amane's parents are from), but I am almost sure no human is born with pink hair.
Is this just a case of "anime hair", and we are supposed to overlook these two hair colors (MST3k style), or is there something else abou these characters?
What about hair color as a recurring element(as @senshin pointed) in other shows?