To solve this question, let's analyze the statements made by each person and the information provided.
Allan said, "I guess it is not black". This means that black is not a possible color for the horse.
Brian said, "It is either brown or grey". This means that the horse can be either brown or grey.
Charlie said, "I know it is brown". This means that Charlie is certain that the horse is brown.
You said, "At least one of you is right and at least one of you is wrong". This statement implies that Charlie is correct because he is certain that the horse is brown. It also implies that Brian is incorrect because he said the horse could be grey, which contradicts Charlie's statement.
Therefore, the correct answer is C) brown, as stated by Charlie.