as far as I am aware this is normal. the one in a neighbours garden does this. but see what everyone else suggests.

I agree with Seaburngirl, my 10 year old Cotinus does this every year.

Its because the upper leaves start to shade out the lower ones - in full shade all day, the leaves wouldn't be purple, so low sunlight levels will cause greening of the leaves. Nothing to worry about though.

In the wild species, and in many other plants, that purple pigment only appears in the very youngest foliage, to protect the growing leaves and stems from late frosts or ultraviolet. Plant breeders have selected for mutations that keep their color longer, but only a few varieties of a few speces keep it all summer in the oldest leaves.

How strange - our mature Cotinus never had anything but purple leaves until they dropped in autumn (though a few did turn red, come to think of it!).

Mine too, Sheilabub, maybe the odd greenish one at the bottom near the centre - mine used to develop red lines that looked exactly like blanket stitch round the edges of the leaves in autumn, really cool... not seen it on anyone else's Cotinus.