Personally, I like your Mera as she is, but if it were me and I was determined to make the change I would take the wig to a professional and then have them recrimp it as well after the hair was highlighted.
As for Isis, Jackie, I LOVE her with darker hair! I never considered her before, but now that I see yours I think I could possibly love this doll with a black wig... but I can't go there. No more Himmies!