Julia, I found this info:

A clematis leaf may appear multi-colored due to several reasons, including green petal disorder (a transient issue related to cold temperatures) and certain varieties naturally exhibiting green markings or leaf-like patterns on their petals. Other possibilities include clematis slime flux, powdery mildew, or even diseases like clematis wilt.

Shirley is correct but at this time of year it is most likely frost damage.
one of mine is showing this and it s 2 weeks since we had a sharp frost.

Ah,thank you both ..I'll keep an eye to see if any more appear.