What color is their bedroom?
I don't think you're locked into neutrals or darks, though of course you want to stay away from Easter pastels.
Not that there's anything wrong with neutrals. Taupe with charcoal grays, tans and creams could be just fine.
Browns with small bits of jewel-bright color would look nice.
I've always loved navy blue, burgundy red and forest green together, perhaps with some gold for accent. (Surely they don't mean no green at all?)
Orange (you know this, right?) is a trendy color for this year. An orange-red with other oranges and reds would make for a bright, warm-toned quilt.
Gold and silver on black is very sophisticated.
For a beachy feel, maybe a sand color with light and medium blues and small amounts of peach.
'60s psychedelic colors are bright and you can even work in some hot pink and spring green with the turquoise, red, purple, orance and yellow.