A: Have you got that dress in another colour?

Possible answers for B:

• B: Unfortunately, not.
• B: The changing rooms are over there.
• B: Can I help you?
• B: That's 10 pounds.

The most direct and relevant answers to the question "Have you got that dress in another colour?" are those that address the availability of other colors. Therefore, "Unfortunately, not." is a direct negative response. "The changing rooms are over there." is irrelevant. "Can I help you?" is a general offer of assistance and could be a response if the store employee is unable to fulfill the request. "That's 10 pounds." is a response to a question about price, not color availability.