It probably never occurred to Lily that in Gensokyo, even teenagers and lolis can have gray hair... Funnier when you remember that both Merlin and Youmu debuted in PCB, which is also Lily's debut.
From the last strip we see that her current suspect list is just the temple crew right now. So it was a bit surprising that she didn't land on Kogasa (because it is a bit bluish) or Nazrin.
It probably never occurred to Lily that in Gensokyo, even teenagers and lolis can have gray hair... Funnier when you remember that both Merlin and Youmu debuted in PCB, which is also Lily's debut.
I once met a girl who had naturally grey/whitish hair. It is actually surprisingly attractive. I guess it only looks bad when you get streaks of grey mixed in with another color.