I bought what I thought was a white plumbago. I even think it was in flower but am not sure. However, this year the flowers were definitely blue. Is this a feeding issue - an acid or alkali issue- or was I just sold the wrong plant?
I've never seen any indication that the color of plumbago can be affected by soil chemistry. The color is determined by the variety of plant. If your heart is set on a white flower, I suggest you contact the store you bought it from, and explain the problem. They should, at the very least, give you another plant. If they don't, at least you'll know never to deal with them again, because any reputable plant store should provide a year's guarantee to their products.