pigment loss

We have an almost 8 yr old Shepherd/Lab mix ( a guess at best ) who over the course of approx 6 months has lost much of the pigment around his gums and lips. We've had him since he was 5, there's been no drastic change in diet nor any illness nor trauma. Has anyone encountered this before? I assume perhaps vitiligo ( we replaced his beloved plastic food bowl with a stainless steel one ) but I hadn't realized it surfaced in their senior years. Could it be a vitamin deficiency? Perhaps supplements could offset this? Anyone anyone?