We have a 10 yr old gray Percheron gelding - whose nose and below his eyes has changed from solid black to a mottled black with pink...seems the pink area is becoming larger this summer. We are south of Houston, TX - with 100 degree days and high humidity. Doesn't appear to be a growth or lesions, just change in color. Any explanations?

This may just be a case of pigment loss (called viteligo when in the skin, leukotrichia when it is in the hair), in which case a cause is rarely, if ever, identified. Horses that don't sweat properly (anhydrosis, also spelled anhidrosis) can have patchy hair loss of the face, though. And of course skin cancer will be unpigmented but should look much more "angry" than what you describe. I would recommend having your veterinarian check this out, though, just to be sure.