What does it mean when a puppy shows lipstick?

When a puppy shows lipstick, it means it is in estrus, which is a cryptic way of saying that a male dog’s genitals are congested. Male dogs in estrus will urinate everywhere and leave their own scent to attract female dogs; they like to climb astride female dogs, or use their front legs to grab people's arms, and rub the inside of their hind legs against the corners of people's legs.
Male dogs do not have a fixed estrus period. Generally speaking, male dogs become sexually mature at 11 months old and will mate instinctively. However, for the sake of the male dog’s own health and the genetic protection of his offspring, it is recommended that male dogs become sexually mature at 11 months old. Breeding should be done after one and a half years old, because male dogs need to be physically mature after one and a half years old.
The estrus of male dogs is not as regular as that of female dogs. Male dogs will also passively go into estrus when there are female dogs in estrus around. If you are an owner who does not want to breed, you can take your dog for neutering surgery at about 6 months old.
If you want to see more, please click: The best time to neuter your dog