That Shades of Grey quilt is awesome -- simple design with a nice impact!

My quilts are almost always very high contrast with lots of bright colors. My best friend always made quilts that I thought were lovely, but, because they were always in muted shades, with relatively low contrast, they just weren't "me." We lost her to leukemia almost two years ago and, among the things she left me was an unfinished quilt top in a Tennessee Waltz pattern. It was done in pale lavenders, washed greens and tans and cream. She had begun hand quilting it and I finished it for her (although I hadn't done any hand quilting in years). Oddly, as I worked with this very different approach to colors, I began to enjoy it. There's a tranquillity to that sort of soft coloration. You might experiment with a small quilt--something you could use as a supplementary gift if you decide later to go for the big one. Good luck.

I love pastels, though I haven't worked with them much.
I'm attaching a quilt I am still working on in pastels (although the picture seems to show
them darker), though I don't think they are quite what your colors are. I am doing an
embroidery design in the white areas.

I made this modern quilt this summer for my niece:

http://www.ohfransson.com/files/new-wave-quilt-1.pdf

She is really into simple, modern designs and loves gray as a neutral, so I used Kona medium gray as the sashing for both of her twin baby quilts. You can do a google image search for New Wave quilt and see lots of variations.

I made the New Wave for an expected twin boy, but she had a girl and a boy instead of two boys! So I ended up making a stacked coin pattern just like I made first for the boy, but using Amy Butler florals in the mix of prints. I really want to finish the New Wave one day, it was so much fun to make. It would be lovely with grays and soft yellows and greens.

Since you asked, make it in the colors that they like. Its always hard to 'work outside the box' but in the end, the quilt is not for you. Suck it up and just buy a jelly roll in pastels and make it. I, myself, would not use a quilt that I did not care for. Remember, its not for you.

The mother may have mentioned to the bride she will be getting a quilt. I think you have to bite the bullet and make one. A very simple pattern that won't be long to make and then send it off to a LA and be done with it.