As a general rule the seedpod needs to look and feel dry and crispy before the seeds are ripe. Another way to tell is if the seeds are wet and slimy. If they are, they probably aren't ripe. Color isn't a good guide, though if they are greenish this is not a good sign. Sometimes you can take immature seeds off a plant and leave them in the pod to dry in sun for a couple of weeks and they will ripen, but in general it's best to leave them on the plant. Why do you want the seeds anyway? Crinums, like most bulbous plants, are much more easily propagated by offsets.