I have always had difficulty growing these from seed. There are 2 problems, I think. One, the seed is very small and hard to collect and two, if you buy a packet it often isn't very fresh and the seeds have a short life.

I was surprised therefore to find dozens of seedlings growing spontaneously in the gravel below some pots where there were flowering plants. What happens is that as soon as the seed is ripe the pod pops open and the seed ( which couldn't be fresher) germinates almost at once. I can just repot my seedlings out of the gravel but if you are growing indoors all you need to do is stand the flowering plant on a high ledge or table and put some pots just underneath, so the seed falls onto them. You don't need to cover them as they are so small. Try it, I'll be surprised if it doesn't work. The plants will be variable in color of course.