@Wende2 I don't do "sheet sets", I've always done mix and match. My place is eclectic and colorful anyway, so it fits the decor. I might use a burgundy fitted sheet, a grey top sheet and patterned pillowcases which incorporate those colors and more, which can hide tiny stains on an otherwise good case.
While I realize many hosts would consider not using "sets" to be somehow tacky, it works for me, guests have actually commented on how much they liked the bedding, and it sure beats stressing over having one part of a set ruined and having to retire the still usable parts.
If you didn't want to go with mixing colors and patterns, you could decide on a color you wanted to use, but have varying shades of that color, like dark grey, light grey, and maybe a grey, or black and white stripe. Then it all still looks purposeful and co-ordinated, and if you see some grey sheets on sale that you like, you don't have to concern yourself over whether it's the right shade of grey.
I do this with towels- guests might get a "set" of towels which consists of 2 bath towels, a hand towel and a wash cloth, all in varying shades of green.
Not using sets also means you can buy separates if they are a good deal- a fitted sheet, or a flat, or some cases.