So if you look at the big scan of their photos, Uta and Risa really have brown eyes, and their hair could be genetically brown or turn browner because of long exposure to UV light in concert and etc.
http://genetics.thetech.org/ask/ask180Many asian have brown eyes, I do too, so I am not very surprise, but my hair is fully black even in the Sun light.
Anyway, it makes them even cuter.