My vet had told me that it was the urine left in the urethra. It goes sour and changes color. Nothing to worry about and he would grow out of it. Lily also had it as a pup, but not after about 9 mos. I think boys have it more often cause they never really "empty" the bladder in case something comes up that they have to mark. Of course, Buster will mark even if he is totally out of ammunition. Seems redundant, but he looks like he's working hard at steaking his boundry so I don't stop him. LOL