Board Thread:Naruto Discussions/@comment-34436445-20180703141210/@comment-1512913-20180716091717

The reason why his eye is considered a Rinnegan and not Rinnegan and Sharingan, is because unlike the tomoe (Sharingan part) the Rinnegan seems to be permanent, like always active. You could say his eye transformed into that state, while Sharingan is something you can turn on/off anytime.

The only time he was shown with the Rinnegan off was in The Last and that might have been just an error.