Last year we planted a variety of evergreen azaleas, some indicas of various colors, some others whose name escapes me. Likewise, the variety of the pinker indica also eludes me (George Tabor maybe?). To my surprise, one of these pink indicas has several branches with clear white flowers. I can see no sign that this is a grafted plant, and can't imagine why anyone would bother to do so.

Interestingly, our nextdoor neighbor has a much larger and older plant with the reverse: a large white indica (G.G. Gerbing, I think) with a single branch of purplish pink flowers. Again, it's a single branch, and it does this every year.

Are these chimeras / somatic mutations? Or, perhaps more fairly, are indicas known to be prone to these kind of somatic mutations, or something else that leads to branches with different flower colors on the same plant?

Sporting is a relatively common occurance in horticulture. Fruit culture, agriculture, and ornamental horticulture have all benefited from the phenom.

So, Purple 'Omurasaki' sported to pinkish 'George L. Taber' which sported to white 'Mrs G G Gerbing.' Pretty cool.

It would be very interesting to know the genetic basis of the color changes. The fact that they are so unstable suggests that they might be due to a transposable element, or something similar, that can insert and delete.