I hypothesized many theories behind his sharingan...
Theory:: Danzo and Uchiha Madara is the very same person or Danzo is a lackey of Madara or otherwise, they both have shown only their right sharingan. It's possible that he gathered the 4 Kages of the 4 ramaining shinobi countries to assassinate them, it may be the moon thingy plan he was talking about when we last saw tobi/madara.
Pros:
1. They almost have the same hairstyle, and since Madara's face is yet to be revealed in the current timeline, it could be that he aged heavily; supporting the Danzo-Madara theory.
2. We all now that Madara is good with deception, he even decieved his own akatsuki by pretending to be a goofy tobi; he may had said that Danzo is a big hindrance to his plans as he told zetsu but it maybe another play.
3. Danzo and Madara have grudge against Uchiha clan, Madara's reason was explained. Although Danzo's was only explained in Sai's perspective, it may be the missing link Masashi Kishimoto have kept in mystery, just like how Kubo does things to make element of surprise in Bleach (It's almost how the patriots manipulated Raiden's life in Metal gear solid 2... xD)
4. As tobi explained Itachi's life to Sasuke, "Itachi was protecting you from Danzo" and "Danzo wants to kill you". It may be just another play to lure sasuke to joining Akatsuki and become his henchman.
5. Madara aspired to become Hokage just like Danzo, and they have some sort of connection with Orochimaru, Orochimaru being a member of akatsuki and as Danzo said, "We must have kabuto first before them, I will be screwed if they get him first" or something like that hehhehe....
Cons:
1. Madara is a full blood Uchiha, it is almost impossible (unless proven) that he have only one sharingan.
2. Danzo wants to kill Sasuke while Madara wants to have sasuke but like no.4 of Pros, it may be just one of his award winning acting..
I'm becoming more and more intrigued with Danzo xD
\m/ rock on