We have a rose bush which is like Black Dragon, but my mother does not like it, so it is a bit neglected. It has recently grown a single pure yellow rose, apparently from the graft, not the stock.
Is there a way to keep this reversion? We like it more this way!

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you can remove all the bits that are that mish-mash mauve. If It's a sucker it will take over anyway.
You could take cuttings from the yellow part to grow on their own roots.
In the sticks near Peterborough
Thank you, nut. I will try removing the stems of the red flowers...
I would also agree with nutcutlet. Definitely take some more cuttings of it too so you can get more of them.