Posted: 19/06/2015 at 21:08
If they are tree peonies, most of those are grafted. If the grafted plant dies, the rootstock may grow and flower which would explain the different colour. While normal (ie herbaceous) peonies should be planted shallowly, that is not the case for tree peonies which should be planted about 4 inches deeper than the pot they came in (or soil mark on the stem if they were bare-root plants.) Doing this will prevent the grafted plant on top being killed by a hard frost and also allow the grafted plant to put down its own roots.
If they were hebaceous peonies, I have no idea how they could possibly change colour!