A: Interesting question. Although we know hydrangea flowers can change color by the soil pH we know agapanthus flower color is not subject to variation regarding soil pH. There are specific instances where variegated cultivars of shrubs, such as Chinese privet, Ligustrum sinense ‘Varigata’, may revert to the solid color invasive parent. But I have not heard of purple agapanthus flowers altering its color. However, I do know the purple cultivars are not as cold hardy as the white variety. I believe you told me yours were growing in pots which made them slightly more susceptible to any cold temperatures. It may well be the purple varieties did not survive our unseasonably cold snap this late spring whereas the white ones have reproduced and thrived.