Jinbe will have grey hair, Chopper will get armament haki (because he’s a doctor he should be able to protect himself) as the member with brown hair. Robin could end up having a weapon. About the hair, her mother has white hair so Robin could have a mixture of both black and white hair, while Brook’s hair might be a wig because he’s literally 90, and it’s not grey. Robin’s official weapon is probably a book like Kuma (or literally Kuma will give her the Bible before he dies). A book fits because she’s an archeologist. Brook’s weapon is his guitar, his voice, and music. With that, all the members have unique weapons. I think this will all happen before the crew meets the last member.
The last person will eat the Paw Paw Fruit then, and it will tie into their position on the crew. I’m thinking boxer would be perfect for someone with the Paw Paw Fruit. There is Ideo who has knee pads who is a boxer, I just don’t think he’s going to join because he is a captain, part of the long arm race, and his black hair. Also boxer is fine because you dont have to hold your weapon, and it’s different from Sanji who just kicks. As for hair color, I’ll go with purple because Chopper has brown hair, so Robin can just have black and white hair which would make all members have different hair colors, which is what Oda might be doing. I think the clue on this character could be that they’re one of O-Tama’s parents, so it could explain Tama wearing the straw hat, but not being the one to join.
3dit: I just thought trainer could be a position for the Paw Paw fruit user, since that’s actually a position in real life. It’s like training animals. Since it ties in with Otama being able to tame animals, her parent could literally be a trainer instead of a boxer. That just means that this character will use the DF as their main weapon.
Edit2: Otama parents being dead is something I missed. It doesn’t mean that this person can’t be a relative of Otama though. If not purple hair then red.