Another forum has this interesting note:
Brian Collingwood, who is very active in the British Clematis Society, and communicates with clematis hybridizers all over the world offers:
"...No, never thought that was possible (clematis changing color), sounds like two plants planted together to me. Sometime when planting small plants, it is easy to plant two with roots and stems mingled together , especially if you grow from cuttings. I've done it myself.
That's all I can think of right now but if any other info comes to hand please let me know?
best regards
Brian