Red leaves (in the sunset)!

The leaves of the strawberry runners I potted up are turning red.  The strawberries still in the ground still have 'normal' coloured leaves.

Anyone any idea why the red colour and is it to the plants detriment?

Cheers,  Tony.
 

My hanging basket strawberries had a few red leaves towards the end of fruiting so I changed from a tomato feed to MiracleGro for a couple of feeds that has a lot of trace nutrients which seemed to work.