1. You planted the wrong seed.
2. Your local bees have been very busy.

Runner beans are profligate outcrossers.

If it is in just one pod, then it is crossed. Very unlikely, but possible.
If it the case with every pod, then what you planted is not what you think it is.

OR: maybe you are mistaken about the color of the seed ! !!
You should've noticed it long time ago.

Thanks to Fusion Power and Seysonn for your reply, it does sound like they have crossed but only on 2 plants the rest are fine. The bees have been busy in my garden this year. I do hope my eyes have not mistaken the colour of the seeds or i might be planting peas instead of beans next year.LOL.

"Achievement" is red flowered, w/purple seeds. If all of the seed from this year was white, that would have been the result of some kind of mix up in seed labeling or planting. If you have mixed white & purple seeds, crossing is the most likely explanation.

As Fusion mentioned, runner beans can cross heavily, especially if you have bumblebees. Even if you grew only one variety, it might have been crossed with a white-flowered / white-seeded variety grown by one of your neighbors.

The cross might have begun with just one seed... but when you planted it, that plant in turn crossed with others in the row. It might be very difficult now to completely eliminate the white-seeded trait from your saved seed. Unless you don't mind the white seeds showing up occasionally, it might be better to start again with pure seed.

If you save seed for a single variety every year, it is good practice to keep each year's seed (dated, in separate packaging) for several years. If a cross first appears, don't save that year's seed, and dispose of seed from the previous year. Hopefully you can recover the pure variety from seed saved two years before the cross was observed.

I have been growing runners from saved seed for years. Gradually you will find more different colours coming in but I would challenge that they are heavy crossers. They stay pretty stable for a long time for me - and that is on an allotment where almost all the neighbouring plots are also growing runners.

Another thing to remember is that the beans change colour as they mature and even after picking as they dry out. I have sometimes thought they had changed colour but after a while they turn the expected colour. White beans might be rather immature. They may be darker later.

Since we only ever eat runners as green beans the colour of the bean is of little interest as long as the pods are a good size and shape and, above all, tasty.

Since we only ever eat runners as green beans .....
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@ flora .... Sure thing. I was just trying to do a search on this "achievement" runner beans, I noticed that almost 90% of the sites offering seeds and info were UK based.

Incidently, I am growing some "Scarlett" runner beans. An do do well here by the Pacific NW. Actually , they are tasty too.

Historically, strains of P. coccineus with white seeds were selected by the Dutch. While the species can outcross with some pole beans, it is also possible that some level of genetic diversity remains in your strain. White seeds are associated with white flower color, too.

Thanks everyone for your comments, I have just been out in the garden to take down the bean row and collect the dried beans that are left on , The bean pods are all about 1 foot long and i have started to remove the beans from the pods, and to my amazement i now find i have pure black seeds in some pods as well as the white ones in other pods.