As a design professional you have been engaged by the client. And kudos to you for winning the work; it is not easy to be selected by a client, who has lots of other options in the marketplace. Your job is to add your particular expertise to the client’s strengths so that you can help them accomplish their project goals. Understanding your client’s intent is essential, and your actions should revolve around helping the client achieve those goals. To the extent possible, focus on using your expertise to help the client achieve their objectives. Sometimes the client will ask you to do things that run counter, in your expertise, to achieving that ambition. In my opinion, you should try to convince them otherwise. In the long run the client will be better served if, during the project, you communicate with them about your reservations and concerns about the direction they have provided. Any discussion challenging the direction you have been provided must keep the overall objective of the client in mind.
Remember that every interaction you have with the client is an opportunity to show them why they hired you: because you have expertise that will help them achieve their goals. If you have diligently shown the client that you do have the necessary expertise throughout the project, it will help establish your credibility when you do have those difficult conversations.