Ballon flower (platycodon) has purple edges on leaves. (2017)

If it happened in late summer/autumn the leaves could have been starting to die off ready for winter.
You say the roots are fine, so I'd wait and see what the new leaves will be like when they emerge in the spring.

I find that mine have a bluish tinge and they are usually fine. think it is just a sign of stress ie too cold as it usually happens to mine at the back end of the year.

Thanks. I got it from a shop so it probably was stress like you said.

if you were able to check the roots does that mean it is a pot? the one I have has a tuber type structure and it is planted out in the garden.

Yeah it's a pot and it's planted quite shallow I just planted it in a bigger container and it is dormant.

It is a tuber type and it's a small type

It's gone out of dormancy looks ok many shoots

That's brilliant news Gardener.

Update: the purple leaves are back despite heatwave. Must be a fungus.