Pink wild primroses?

I bought and planted some wild primrose seeds about three years ago, and since they have seeded themselves everywhere and flowered every year. Previously they were always pale yellow. This year for the first time there are a few pink ones along with the yellow ones. I don't have a lot of different coloured cultivars, though I did have one purple one I bought in Morrisons, which wasn't as vigorous as the wild primroses, and hasn't flowered yet this year.

I don't think these pink primroses are the result of hybridisation with cultivars, but I could be wrong. They seem like a genetic result of several generations of yellow primroses.

Does anyone know why there would suddenly arise pink primroses from yellow primrose seeds? Has this happened to anyone else?

 

Definitely crosses with the Polyanthus type primulas. All you need is a strong flying bee and there you go.