There's no way you can do anything to change the colour of the flowers. The way they look now is just what you've got I'm afraid - many multi coloured flowers (especially more recently introduced ones) can revert to one or other of the original parent's colour, but if you keep it another year, you might find its different again... its very attractive anyway, as it is.

They don't normally turn color like that. It looks like the original plant has been replaced by a self-sown seedling, or may have had a mutation, called a "bud sport", on one of the branches, which has grown faster, and taken over the plant.

Bamboo, thank you for your advice. I do like the muted colouring, but the original was just so flamboyant! How do the growers ensure constancy in new plants, as they must if they continue to market them every year?

They usually grow and regrow for 3-5 years or more to check consistency of the genetic information, or at least that's what they used to do. Given the plethora of new colours/variations in various plants (not just Osteospermum) and an observed somewhat increased tendency to colour change, I rather wonder whether they've shortened the testing time! Certainly, my sister was really annoyed when her new, beautiful, peach coloured rose turned pink the following year, and that was a new variety at the time...

I've had an osteospermum for 4 years, managed to get it through winter each time - not sure of the varietal name, but its the one with white petals, dark blue undersides, and a dark blue centre, and that hasn't changed at all - but that particular colour variety has been around for quite a long time, some years in fact.

They don't actually 'change' color', but the actual colors can be enhanced or muted depending on how much sun they get, soil composition, etc.

It happened with a day lily that I divided. The division I planted in the back that gets partial sun is very pale pink almost white, but the one in full sun is strawberry pink with orange highlights. You can see both examples in my gallery for you to examine for yourself. You wouldn't think it was the same flower, but it is. Just enjoy them, don't over think it. The season is very brief.

On closer examination of the original pot, two of the flowers showing are solid orange, rather than with the purple aura. I think that what happened in this case is that the minimum wage worker responsible for moving the young plants from flats to the pots, three or four plants per pot, grabbed a cutting from the next flat over, which was a different cultivar. Over time, the plant with solid orange flowers took over.

I hadn’t realised it was so complicated! Thank you, everyone who responded.