What variety of clematis is it? Some do have green flowers.

If your clematis is not meant to be green flowered, its one of two things - there is a transient condition that affects them when the weather is cold, and you may see green streaking or green petals on the flowers. This should pass, so later flowers will be unaffected. If though, the second and subsequent flowerings are completely green,and more, if they're slightly distorted, then your clematis has a condition called (unsurprisingly) green flower disease - it does not pass and is permanent. So only time will tell whether its a passing thing due to the chilly temperatures (most likely) or whether this particular clematis needs to be dug up and disposed of.