why foliage changing colour?

Why is this clem's foliage changing to purple? I think its Sylvia denny.

 

I know of two things it could be, but it may be something else......

Is it in a draught or a windy place ?

Has it been taken from a warm place e.g. a greenhouse and planted out without being properly hardened off.

 

Leaves changing a red colour can also indicate a mineral deficiency.
http://www.gardeningknowhow.com/plant-problems/environmental/leaves-turning-reddish-purple.htm

 

As above with well drained soil. Perhaps a slow fertiliser like bonemeal. Once it gets established it will probably be ok. I have quite a few here set into the soil. I use a bit of farmyard every other year and a touch of Bonemeal.

 

It has been out for around 4 or 5 weeks. It has had no drought to mention and although garden is pretty sheltered it has been very windy this spring. Its in peat based GP compost with a touch of horse manure mixed in. Polystyrene drainage.
It does seem to be growing well, we'll see what the warmer weather brings, if it ever comes!
PS Its neighbours, Dorothy Walton and carnaby, have the same potting medium but no change of colour on leaves.

 

I think some varieties have a natural purply/reddish coloured foliage.

Tbh, I don't know which varieties do, but this could be one of them?