Just throwing my 2 cents into the discussion of Rena's hair.
In Rena's case, I feel the black hair is vital for her image/character.
I agree with this. Rena's appeal is rooted on her 'dark hair, fair skin' almost princess-like image - she's a classic type of beauty. And although light hair would also look good on Rena, I think silky black hair is far better suited for her.
2. That black hair policy came out of nowhere. Not sure who/why that all of a sudden became a thing for SKE48. Can't complain though because black hair > all.
You know I've always wondered whether they had this unofficial rule of keeping black hair while in the group (as some people have also speculated before). Yuimin, Haachan, Imade... the first thing I noticed when their first few post-graduation pictures came up was that they all seem to have lighter hair. So yeah, I thought maybe they had restrictions when it comes to their hairs. Like you said, though, I don't really have problems with it because black hair trumps all.