If that's your goal, you probably shouldn't call the class bg-blue? It should be bg-blue-but-purple-on-mobile. But then it's definitely so specific it's the wrong way to select a style. If you want the page background to be blue but purple on mobile, write that in your CSS with a selector of "body" instead of ".bg-blue-but-purple-on-mobile"

The point being you can't do this with inline styles: "bg-purple md:bg-blue"

Yes but you should not use inline style for that (nor Tailwind).

The whole point of Tailwind is that it lets you do that. You can debate if Tailwind itself is a good idea but thats not relevant to comparing it to inline styles.