Whenever you are selling always remember that line. This is what separates average sales people from superstars.
Customer is always right do not mean literally that in every situation the customer always speaks the truth. What it does mean is that salesperson's opinion of a product or a service does not matter. It is given that it is in salespersons best interest for a customer to buy something. The only difference between a sale or no sale is customers will to buy. Therefore, it is only buyers opinion that matters and a salesperson operates within opinion constraints of a customer.
For example:
Pretend that you are selling pens. Your customer is looking for a blue pen and picks up a black pen. When a customer selects a black pen he/she is happy with it and exclaims "What a great blue pen!". Would you disagree with a customer and tell him or her that it is in fact a black pen? This will cause a conflict and you would likely lose a sale. A definition of "blue" can be different from person to person. Even with something non subjective as colors, what if a customer is colorblind? They could looking for a blue pen that is in fact black? In this example, a best approach would be to agree with the customer and comment that while it is an excellent blue pen, most people will perceive it as black. With this approach you are still within the bounds of a customers opinion and, therefore, customer is right; however, you do provide an honest service warning that others may perceive a product they are buying differently.