There are going to be many different factors affecting the appearance of flowers in photographs. One thing that stands out to me in the photographs that have been previously posted in this thread is that the direction of the light source in relationship with the flower is very different. In the picture that Peter posted, the flower is backlit and in the photo posted by Ron, the light source is in front of the flower, most likely a camera mounted flash. Both of the photographed flowers appear the same to me.
There are however several different clones of U. graminifolia available, the flower variation between clones is definitely genetic. I am not sure to what extent conditions affect flower coloration, my guess would be that under extremes there would be an effect; however I have not noticed any effect on the plants I grow.
The flowers pictured below represent genetic variation, both plants flowered at the same time and were growing next to each other in the same conditions. Both pictures below were taken on the same day, using the same camera/multi-flash setup.