It's a little like hydrangeas, they can be pink or blue or white, depending on the soil.
For my regular white ROS, the seeds came from my great grandmother's yard and they had only ever bloomed white with the maroon center. The second year I had them, there was a pink bloom or two on the tree along with the white. The only difference I could think of, and all that others ever told me, is the soil difference might play a part in it. Now, after many years, it has many shades of pink and the white is still there too.
I haven't researched it to any great length, I just enjoy whatever it gives me, so I really don't know.
