I have a horse. Do you know what color it is? Allan said, "I guess it is not black". Brian said, "It is either brown, or gray". Charlie said "I know it is brown". I said, "At least one of you is right and at least one of you is wrong." What is the color of my horse if the color is one of the above?

Test each color against the statements. If black: Allan is wrong (said not black), Brian is wrong, Charlie is wrong - violates 'at least one right'. If brown: Allan is right, Brian is right, Charlie is right - violates 'at least one wrong'. If gray: Allan is right (not black), Brian is right (gray), Charlie is wrong (not brown) - satisfies both conditions.