It's possible they change with age, but it might be sunlight. Mauve in the leaves can mean the light is hight (but not enough to burn). Some phals are more prone to that anyway, but others only show it in bright sunlight.
Thank you for helping. The mauve is not in the leaves but in the flowers. It adds to their exquisite beauty, I think. Just to be on the safe side I have moved it out of the sun. My Yu Pin Pearl has not changed colours. I am not complaining as the sunshine is so wonderful and welcome after all the dark winter months. Also it is great for my 21 plumtomato seedlings which came up 4 days after sowing. My Impatiens New Guinea cutting also seems to thrive in the sunshine.