I have two different varieties of the flower, portulaca: one is bright pink, the other is pure white with pink specks at the centre. Recently a flower bloomed which had 50% of the former and 50% of the latter: both traits in the same flower. Is this an example of co-dominance or is there some other reason for it?
This can be a result of a somatic mutation, especially if the other flowers on the same plant don't have the same color pattern. Somatic mutations are not inherited for parent organisms but occur spontaneously in one of the cells in the body. If that cell then proliferates all of its descendants will have the mutation and new phenotype associated with it.
Look for example at Fig. 1 (c, d) in this article. Those peach flowers have similar coloration to the one on your photo.