You mean the exterior is dark gray? That would make the color "Galactic Gray Mica"
No, SR5's don't come with leather interiors, leather was either added at the port when the car came into the US, or the previous owner decided to add leather to it. The color really depends on which interior color the car originally came with. If it originally came with the Taupe/Tan color, then they would have most likely put in the tan leather. Does the leather color match the seat controls and the door panel? It would look a bit weird to have an interior with gray leather seats, tan carpeting and lower dash panels and a black upper dash. Check the label on your driver's door jamb. There should be something like C/TR: 1E9/FG42. If you get FG42, the car originally came with the tan interior, which is why somebody decided to put tan seats in. If you get FG12, the car originally came with the gray interior (not sure why somebody would put tan leather in a gray interior though).
Hope that wasn't too long!