Those inside toes are the ones they primarily use when grooming and scratching. It's not uncommon for them to be a little more red then the other toes, or even a bit bald from the fur rubbing off. Nothing to worry about unless they look bright red (or other color like purple or black) or scabbed which would be a sign of an injury.
My chin sometimes has little faint patches like that on her toes where the fur is a little thinner. Since it never seemed to spread or get worse, and sometimes will disappear for a while, I always just assumed more or less what Amethyst said: bits of fur get rubbed off as she uses those toes and feet. It doesn't seem to cause her any discomfort.