If this is for a graduation quilt, I have made them using the school colors. Having made some hard-used baby quilts, I'd probably not use flannel as the fluff tends to disappear or doesn't fluff any more. Too, flannel tends to fray so seams may come apart if it isn't tenderly cared for, If this quilt will be laundered repeatedly, you might consider how it will look as the colors fade with life as the quilt will know it. I've made some using black and, with use, they look pretty dull. Speaking from experience here.

I commend you for wanting to give a lasting gift. No matter how much time and thought you put into a gift there's always the chance the receiver won't like it or take care of it. Make the quilt that demonstrates how much you love her and that way you know you've done your best. What the receiver does with a gift shows who they are.

My oldest granddaughter was a slob when she was in high school. I made her a kaleidoscope quilt for her graduation and much to my surprise she still has it. I also made her a baby quilt and a wedding quilt, and she says her graduation quilt is her favorite. She keeps a lovely home now that she is married with 3 children. You never know with teens.

I just made my niece a Disappearing 9 patch when I found out quite suddenly that she needed heart surgery. She loves it.
It looks complicated but is super easy to make.

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.

Welcome to the QB from SE Michigan! I am so glad you jumped right in and are enjoying all the fun! I found my teen aged daughters, back in the day, liked things that were two toned like white and black or purple and white. They were more concerned about the colors than the design. I used a good cotton thread to hold the pieces together and that helped a great deal. My youngest daughter returned one of the quilts I made her as a teenage back to me recently for a bit of repair. The quilt is holding but the batting has slipped. I will 'someday' get around to fixing it. Oh did I mention they are both in their mid 40's now so the quilts have lasted about 35+ years.
There is no right or wrong talk to her get an idea on colors ideas and so on then go from there.

I like batiks for young people. I used Jordan Fabrics Jelly Roll Drag Race for a male grad last year. This year I like her new pattern with the diamonds.

D9P is a great idea, Susie. They do look complicated but aren't are go together so quickly.

Teen quilt
 

I will certainly get ideas as to colors and pattern. Thanks for all your suggestions!