Tracy,
Hi and welcome. You will surely find much satisfaction in your efforts in the future. According to others who have children involved, they will be the better for it too. Thanks for joining our efforts. Spread the word - tell anyone who has an ear to listen about AS.
Color of bedding? Any color that won't bleed (reds in particular might). It's my opinion that the sheets in particular should have some cotton or other natural fiber content for comfort, but that's me - nothing written in stone. I prefer a minimum of 45% cotton if they have polyester content. I mostly get all cotton, but that's me. I did get one set on clearance this week at Target that had some polyester in it because it had a car print.
Which brings us to the design. We have had requests for cartoon character bedding, dark-colored sheets, and everything in between. If they want sheets, they more than likely won't refuse anything clean and comfortable, even gently used quality linens, which some of us have sent. Some of the outgoing units will offer their bedding to their replacements if they have life still left in them.
Blankets: anything that would keep out the chill of the night that is washable. They prefer not to have wool because they can't send it out to the laundry like cotton and synthetics. Some of us have even sent decent throws that help their beds look more like home in areas where it isn't extremely cold.