There are two things that can cause green flowers - one is clematis green petal, caused by low temperatures, and it tends to affect clematis with pale coloured flowers -it usually resolves as time goes on. The other one is Green Petal disease, but the flowers are usually distorted as well as green, and that is caused by a phytoplasma - affected plants should be removed and destroyed. Further info in the link below, scroll down to the right part

https://www.rhs.org.uk/advice/profile?PID=773

This is an unstable variety which often reverts. There is more information here:
https://www.taylorsclematis.co.uk/clematis-sieboldii.html

Thank you both. Maybe it will revert to the right colours next year though the pale green is quite pretty. Different anyway.