First of all, I have never heard of a montana clematis that has purple flowers. Are you sure it is a montana and if so, please find the name of it since I think a purple montana would be marvelous?

If the seed were still green I would be a little leary of them being mature. Generally speaking clematis seed start turning from green to yellow and eventually to brown. If still green, I would assume the seed was not mature. Once the seed starts turning yellowish, its connection to the mother plant is being lost, hence the color change from green to yellow. I have picked plenty of pale yellow clematis seed for others to germinate and they haven't had any problems with getting them to germinate.

Assuming the seeds are viable, you can start them at any time. The herbaceous clematis seed are warm weather germinators however most of the others prefer some sort of cold stratification to get good germination. The ones I have started in the past were potted up in the fall and kept in a cool area over the winter and either germinated during the winter or in the spring. If you do pot them up now, don't be surprised if nothing germinates until next spring after they go through a cold stratification period.

Thanks for all of the good advice. I was collecting seed because lots of them were falling out of the flowered heads naturally, but I think I will continue to collect over time if it doesn't all drop. I also took some cuttings and rooted them in my greenhouse, so we'll see how that works out.

As for the color, I did unintentionally mislead you: my wife says it's more a deep pink-magenta than purple (I'm somewhat colorblind).

No problem Kudzu. I know everyone sees colors a little differently but thought that if there was a purple flowering montana out there I would love to know what it was! Thanks!