There are lots of heirloom varieties that come true from seed, so this can't be true. They are self pollinating, but also by bees, so some mixing can go on in the garden. Maybe that is what you read. Or if they are hybrid varieties, they would not come true either.
It is much easier to save seeds if you are only growing one variety because you don't worry about mixing. I wish I could tell you an isolation distance where you could grow another variety and not worry about cross pollination.
Remy

Thanks, I think you are right and I will just sow them later this year just to use them up as they are so pretty.

I seem to recall my grandad commenting on his multicolored (but his favorite color in the garden was red) sweet peas all going pink (which he wasn't so fond of), but Papa was notorious for saving seeds forever. Like 15 generations worth of the plant. So, after 15-20 years of recycled seeds, I think they did eventually revert, but then maybe his memory after all that time wasn't quite right on? Dunno... but he's gone now, and just remembering his complaining about this made me smile... LOL

Amanda 'romando'