Hi,
Wait for them to turn color. Green in seeds is an indicator that they have not matured all the way.
Remy

My approach to this is totally lazy. Let the plants die back by frost or whatever and a little later go out and collect the seeds off the ground. Some I also push a bit into the ground to self sow. I am not sure if there are losses to critters or whatever but I grow so many I can find enough seeds. Next year these are my suprise nasturtiums because by the time I get the seeds I usually cannot remember what plant they came from.