The green form is generally hardier, though as they are seed grown, they can vary from plant to plant on exact levels of hardiness. This last winter has prompted a huge response on this site from people asking about frost damaged plants, many established to large size because of the run of generally mild winters we have had over the last twenty years or so over much of the UK.

In short, hardier if planted in the ground. Green form hardier than any colour variants and if you live near the coast (any part of the UK) you generally will not need to worry.