This will be a "because that's what the puppy does answer"...the new growth always starts out red and as time goes on turns green, why, because that's what the Photina does.

they also do better out of full sun. Sun has a bleaching effect on some colours of foliage.
welcome to GoY too :o)

Some sun is necessary for the color to develop, too. Also, heat will reduce the color, but it would have to be extraordinarily hot in the UK for that to happen.

If, on the other hand, the tips of the leaves are going brown rather than just losing colour, if your plant is potted, insufficient water will cause that.