^ Being red-green colorblind, I always have an app on hand for this... and apparently her hair has an RGB value of 156, 174, 161. Which is pretty close to grey (for various shades of grey, all values would be equal), but it's also slightly greenish.
Now, if I was to speculate as to why... I couldn't. Art is far too subjective. Best guess is the artist thought it looked good.