Answer:
The phenotype of the sneaky neighbor dog will be pink. Let's consider the phenotype of our spotted gummed Labrador to be CPCB and neighbors dog to be CPCP
CP CB
CP CPCP CPCB
CP CPCP CPCB
The results of this cross show that there will be a probability of having pink dogs as well as spotted dogs. Hence, this proves that the neighbors dog had pink gums.