Brown

Best picked when fully ripe and brown. Sweet pea seeds are worth saving from healthy plants as they are 'colour fixed' and usually grow the same colour.

I thought that unless they're species sweet peas (like Lathyrus matucana) they're unlikely to come true from seed? Never tried it though as I prefer to not to let mine seed to encourage more flowers.

Green sweet pea seeds will rot, so leave them as long as possible to mature.

i agree with above, brown and dry is best, i tried when green and got nothing, so waited and got lots of seeds