Several years back, I had a pink lupin, and wanted to gather seed from it, but was told that it would revert back to bluish purple. Well, that plant died years ago, and this year a volunteer came up and it's pink! The other lupins seed all the time, and I'm always getting volunteers - this year I even got some that looked like a combo of purple and pink (quite pretty) so I'm a little confused...
It's normal to get some color variation when you have plants reseeding like that. Probably the person that told you your pink would revert to bluish purple was just trying to say that the seeds weren't guaranteed to come true, and since bluish purple is the more common color that there's a good chance they'd go back to that instead of pink. But you can get seedlings in a population like that which will end up a different color. Same deal though, no guarantee that seeds from the pink one would come up pink.
You never know, you could get some pink seedlings from it so it's worth collecting seeds and trying, just don't be disappointed if you don't get pink babies. I don't know how easy it is to control pollination, but if you can try hand pollination to make sure that the pink plants only cross with each other and not with the purple ones, then over time you might end up with more pink ones. They still may not always come true, but continuing to cross pink with pink would probably over time result in higher percentages of pink
The original one I got as a plant at a nursery, that was pink - that was many years ago, and all I got from natural reseeding since, was purplish blue, and this combo of purplish blue/pink (that was quite pretty) then suddenly I got this new pink one again!