Fairly common for early blooms. I've even had purple clems do it.

Yep seen the same thing on several clematis that are exposed to cool temps. during bud development in the early spring. Have even seen it on clematis that flower late in the fall as well.

Another possibility is that you don't have what you think you have. Of course, if it has bloomed with the appropriate color previously this wouldn't be the issue.

I have others that have early blooms that are green but the unusual thing is that Vienetta stayed green (and growing) all winter and bloomed in early February. Usually it dies back and I prune it to the first bud unions. I didn't word my original post to mean what I intended. The first blooms of Duchess of Edinburgh are always part green and sometimes almost entirely green, sometimes greener than the bloom on the left here:
{{gwi:578773}}