I've never seen a purple teabag, but now I want to

I’d argue yes. What people care about more is minimum steep time and minimum leaves by weight per mL water. You can use the brew ratio for this to actually define your tea to a standard like black tea. Though you’d have to define brew ratio which I trust the British to do.

After you define those things though, you’d probably measure the amount of liquid and kind of ignore the weight of the tea and steep time, so long as they go over the minimum per serving. Unless you want to argue that adding more leaves/steep time means that you’re consuming more tea, which seems wrong.