Lets start with Aokiji, who we meet the first time at Long-Long Island. His name translates to Blue Pheasant and he has the ice-ice fruit.
The next admiral we meet is Kizaru, who we meet at Shabondy Archipelago. His name translates to Yellow Monkey and he has the light-light fruit.
The last admiral we meet pre-timeskip is Akainu, who we meet at the Marineford war. His name translates to Red Dog and he has the magma-magma fruit.
Now after the timeskip Akainu became fleet admiral, Aokiji left and 2 new admirals were drafted Fujitora (Purple Tiger) and Ryokugyu (Green Bull). We meet Fujitora (Purple Tiger) for the first time in Dressrosa.
We dont know wether Ryokugyu (Green Bull) is male or female so for this part i will adress him/her as he/him. Lets use the same format again (seperating the names) and start with the ‘Green’ part. I’ve seen a couple of theories that link his power to either trees (nature), radio-activity or time manipulation. But when i think of the colour green i think of healing (mainly because off all the hours i spent playing Pokemon when i was a kid). Because of this i believe his function before becoming an Admiral was a doctor, probably a Marine one. We know that the Marines have a Medical Centre.
If Chopper used his research for the rumble balls then it is likely they have a similar kind of Zoan devil fruit and that is the human-human fruit. I believe Ryokugyu (Green Bull) has the human-human fruit model: Monster/Berserker/Hulk.
With it he transforms into a monstrosity with insane physical abilities but he also looses control of his mind (like the real hulk). As an Admiral i believe he orders all his troops as far away from him before he activates his devil fruit (this would give some cool panels). The drawback from this might be that he can’t use Haki while in berserker mode which would make him useless against logia but still insanely effective against paramecia and zoans.