In a past rp I was in,  there was a guy way too obsessed with being the 'special' ranger of the group. If you wanted, who gets green/white could be based on someone's performance as a ranger, rather than just deny everyone outright. Or, follow the show and have the Green Ranger be evil, with cause and motivation determined by yourself. You haven't mentioned Rita yet so I don't know who the antagonists are. 

I can give up Yellow to make Black. I just didn't expect many girls or people that would rp girls to hop on this.