Some variety of Datura only have green seed pods, they don't turn brown. Watch the scored lines on the pods, they will start to split open if they are ripening.
As long as the seeds themselves are brown they usually are viable. Sometimes if you pick the pod and keep it a couple of weeks 'as is' the seeds will ripen more and then you can break it open and save them. lay them separated on paper towels for 3-4 days until the harden a bit and feel dry, then place them in an envelope, seal it, mark the date and the content's name and keep in a drawer in your fridge until you are ready to plant.
Good luck!

Don't worry about a freeze. Datura seeds need stratification (a freeze and thaw) to germinate well. Seems to me that, in past years, a frost would promote splitting, but not this year.

I picked my purple metal yesterday. Pod never split and we've had frosts. The flesh was very soft, but when I cut it open the seeds look good. I'll let them dry on the counter and put them in the crisper drawer in the fridge as Ruth Ann recommends.

Some but not all Datura will do OK with freezing temps.
I live in Zone 5 in Canada and I regularly have Datura stramonium self seed here but that is the only one. The doubles don't like to get frosted much in my experience. You may get the seeds to germinate if they are fairly fresh but not so well if they have been saved even properly over the winter months.

thanks to everyone for the replies