D. T. Green wrote:
Some mint i got from the supermarket i rooted and planted some in pots
outside. A little later and some of the leaves have turned a dark reddish
coloured brown.

Does this mean they are passed their best for picking and freezing? Also
how is it best to take the leaves off and still keep the plant alive for
next year please?

Not an answer to the question you asked I'm afraid, but do be careful
planting mint out in your garden, as it can run rampant - we have mint
in the garden that the seller had just planted before we got here, and
we have never managed to get rid of it! (We also planted black peppermint
ourselves, but in a sunken pot, and it's stayed where we want it)