The rectal skin can be pinker in color than the surrounding skin but should not be red. It is possible that your kitten has a small rectal prolapse--the inner rectal tissue protrudes out of the anus. This can become problematic--this skin is not supposed to be outside and is sensitive to dehydration, infection, and irritation. Sometimes small rectal prolapses can occur right after defecation and resolve within a few minutes.
I would have your kitten examined by a veterinarian to make sure this is not anything serious. If all else is well with your kitten this can probably wait until the next vaccine visit.
I would have your kitten examined by a veterinarian to make sure this is not anything serious. If all else is well with your kitten this can probably wait until the next vaccine visit.