It was fun researching this, Robyn! It seems that "pig face" is used as a name for a wide variety of what we call "ice plants", here in the States. Do you know which species you are working with? If it is one of the annual species, such as Livingstone daisy (Dorotheanthus bellidiformis), one of the possibilities is that the local pollinators preferred the white and yellow ones, so the other colors didn't get a chance to reproduce. This happens to us with African daisies (Dimorphotheca sinuata hybrids)--they start out as mixed colors, but after a few years only the orange, or occasionally the yellow, ones are left. If they are a perennial species, other factors may be at work.