I have no idea. Kalibos is a long season oxheart type not well suited to my growing conditions so I am not very familiar with it. If you got new seeds or new plants this year, I would suspect variance due to impure seed. Happens often with OP's since certification has been phased out. Sometimes redness in brassicas is proportional to sun exposure but that shoulf affect the outer leaves first.

Thanks for your reply farmerdill

Not light exposure as we've had a very sunny summer here. Could be a "rogue" seed - but the outer leaves are very similar in shape and form to the other cabbages in the bed (which do seem much redder in the head). Not to worry, it's all shredded in jars now and the spiced vinegar is heating :)