Who said it's red? I thought it was at first to, but if you look at the most recent chapter, when Sasuke uses his eye to see limbo madara, the colors change, it becomes inverted. Now look at that inverted color picture, sasuke's sharingan eye is red obviously, it looks dark/black in the inverted color, but his rinnegan eye is light, so it's a different color than his sharingan eye, so I don't think it's red but probably the normal rinnegan color, at least that's what I get out of that frame. I want it to be red though, would be cooler in my opinion.