When you look back 4 years from now, I think you will be surprised. A graduating high school senior is MILES apart from a college graduate. They change and mature a lot between 18 and 22. If you take her on a day trip to pick out the colors and the design, this will give her a wonderful memory of not only the time she spent with someone she loves, but a lovely momento as well. She will be able to take the quilt with her as a reminder that during tough times there are people who love and care for her. More than likely, she will want a new one anyways because her tastes have matured.
My kids took their quilts with them to college. It brought great joy when the quilts came home well worn, faded and in need of mending. Why? Because it was proof positive that the quilts were being used and hopefully brought them a little bit of warmth and comfort from home. Although I have replaced their college quilts with new ones, they still cherish those quilts because they are more than blankets, they are memories.
When I gift a quilt, my expectation is that they will be used. When quilts are used, they get stained and faded from washing. That is how memories are built. Besides, I can always make another one.
I am sure your grandchild will love and cherish the quilt you make for her for the rest of her life. She may not today, but in a year or two, she will.