I think it's implied, but unstated, that this guide is for picking a palette for an app that follows modern design trends. The choice of a slack-like UI reinfor

I think it's implied, but unstated, that this guide is for picking a palette for an app that follows modern design trends. The choice of a slack-like UI reinforces that. You can get away with many fewer colors if you use different design language, but very few (not none, but few) apps are built with those sorts of aesthetics.