This will depend on the type of poppy. Many of the big showy oriental poppies can have sterile seed, but others, like Iceland poppies seed themselves around and do not necessarily come true to colour. Californian poppies, the red ones, also seed themselves around and do, in my experience, come true to colour. But others may find differently.

The Welsh poppy Meconopsis cambrica seeds everywhere and is always yellow or an orange/yellow. Of the blue Mecs. some have sterile seed and others fertile seed they always come true to colour but can, in some cases, cross with each other.