This is part 1 of this week's GNO series.
Cool Blue is an almost-periwinkle blue, while Metal Blue is a deep, greenish navy - kind of like slightly oxidized titanium metal.
Judging from the swatches below, it seems that Metal Blue would make a good upper lid liner, while Cool Blue would be great for brightening up the lower lid.
I was half right. Metal Blue looks great as a top liner. Cool Blue was extremely difficult to apply and spread all over the place, though.