It depends. Your capsule will be all neutrals, and "must-haves" are often trendy prints or colours, or a style that will look out of date quickly, so they don't belong in a capsule wardrobe. But there are exceptions. For example, if you were looking for, say, a trench-coat to fill out your capsule, you might decide to pick one that had a trendy cut or details. Or maybe you really like the style of cardigan that's out this year, so you buy one in a neutral colour as a basic that you'll keep for many years and one in a trendy colour that you might get rid of sooner (though if it looks good on you, maybe not!).