Dark on top, light on the bottom (unless you have big ol' hips). This also links to a neat tips & tricks piece from some "style influencers" in which shirt stays are called "shirt garters" which is what I'd call them because America.
everybodylovessuits: “Dark suit, light grey vest is almost always good combination. If you buy a vest get a light grey one. It will work perfectly with black, dark blue, dark grey and light grey suits….so pretty much it will work with every suit you.