The nature of Mystery Quilts and why I don't do them until I know what they will look like.....kind of looking at the end of the book before you read it.
I've seen many quilts made with the same Mystery Quilt pattern, some just look faded because the fabric chosen just blended in too much and the pattern was lost, others a bold fabric in the wrong place wrecked it. All of them call out lights, mediums and darks....but some of that can be a matter of what you think are enough contrast between them...and the inability to make a sample block so you can see if you like the look or if a different fabric choice would work better can make or break a quilt. That being said, I've seen people who knew what the pattern was and would have chosen the same fabrics regardless.
I've only done one Mystery Quilt, and the one teaching it used our small group (she was a member of it) to test out the pattern to see if we would have any problems with it at the retreat. She looked at my fabric choices before hand and suggested I might want to replace one or two of them....I did and the quilt turned out fantastic.
Very mean spirited of the instructor to just smile and turn away. I think that I would never take another class or do a project with that kind of a teacher. The aim is so that you have a successful project, not to give her ammunition to point out where you could have made a better choice for fabrics.
I'd like to see all Mystery Quilt instructions suggest that you might want to save your "favorite or featured fabric" to use for the #____ (fill in the blank here) fabric to showcase it best.