Hmm.. i think it's a matter of opinion. Here's mine:
If you use the same colors, stamps, general layout.. it's a "Case".. if I use their ideas and make them my own I call it "inspired by...."
There's a fine line, but then as long as you acknowledge where you got your idea, I don't think anyone around here would complain at all.. afterall, we're here to share ideas
Peace!