I have had three large pots of healthy colorful geraniums for several years. Then one summer they were all red. What happened?
Your geraniums mutated to a different color. Sounds like science fiction but it really is just science and fairly common in the plant world. In fact plant propagators are always on the lookout for changes in flower and foliage color, growth habit or other features. When something unusual or better is discovered cuttings are taken, offspring propagated and new plants introduced. That was the case of the golden delicious apple.