I have been intermittently supervising mint leaf collecting and processing.  It is one of the various culinary plants that Mum and Aunty Biscuit have at their establishments. Left to its own devices it runs riot so Aunty Biscuits has  recently moved from remnants an old chimney pot into the mortal remains of a clay pot inset into the ground whereas Mums is in a proper pot. I don't think that it is used for making dyes although I could check on woofapedia.  At a base level you sling a sprig in with some new potatoes, boil them and eat the potatoes.  You can also make mint tea.  In this instance though they were making a base stock for mint sauce.  This seemed to involve stripping leaves from the stalks (presumably they had been cleaned, probably whilst I was checking the pond for frogs), chopping the leaves up, packing them into steriised jars and adding pickling vinegar.  They use pickling vinegar as its stronger.  At time of use some of this stock mix is diluted and has sugar added.  In the middle proceedings Granny Biscuit rang.  She says she freezes the leaves whole and then they break up when you take them out to freezer,