I used to judge baked goods for 4H'ers. Some would be really awful, but I always concentrated on what was good about their product. For instance, for a dinner roll that was raw dough inside I would say "You did a beautiful job on judging the color that the rolls should be!" and for one that was burnt on the outside I would say "What a great job you did making sure that they were all the same size!"
I did once read an article on this topic. Apparently it's: Compliment - Critique - Compliment in that order.
You can use the same tactic here. "What a great job you did on choosing the colors! Did the batting manufacturer have information about how closely it should be quilted? I'd hate for it to get lumpy when you did such a great job on it."