P. somniferum is the annual one. I am sure you can buy seeds of individual colours. And if they are properly pollinated (human rather than bee) then yes they do come true. I have a friend who used to supply seed of different forms and colours. sadly he has given up the business now.

I grow these every year from seed Scottish, they can be bought in GCs in individual colours. I've saved seed from my plants and the colour comes true. I scatter some seed around in Autumn and some in early spring. The seedlings are quite easy to spot as they are a greyish green and flat to the soil. They make a good filler plant for borders as they don't take up much room.

you can sow now and you will get flowers this year. I started with plum ones and over 6 years I get a range of colours thanks to the bees.

Thanks all - I'll pick up a packet of seeds when I visit the GC this weekend.
Owdboggy - yes I realised they were annuals after looking for them online, which was why I probably missed them when I was looking in the GCs...Doh!!
Jaykaty - thanks for describing the seedlings, I'm a bit obsessed with weeding - too much time on my hands I think!!
SBG - thanks. I saw the spring sowing advice online and thought maybe I was a bit too late. Mind you - still very much spring up here ;)