If one really cares a lot about finding white or pale flowers on a solid coloured individual, I wonder if it could work to get backbudding all over the plant.
A plant may be a mosaic of tissue with most of the plant made up of patches that would produce purple flowers.
But there could be patches that would produce the pale flowers if a branch were to grow from that patch.
However, if a grower is know to produce mainly, and possibly only, solid purple 'Issho no Haru', this won't work.
It may work if the cutting that has only shown 'bad flowers' does come from a mother plant with the proper flowers.
Another way is to look at pigmentation of leaves as they lose chlorophyll.
I have two new whips that show only 'bad flowers'. I will observe to see if there is any part of the plant that has signs of being able to produce the correct flowers.
'Hanatsuzuri'
Picture from last year. Surprisingly, this azalea is fertile both as a male and a female despite flowers with thick lush petals.
It came from Japan as an unverified whip and the patterning was absolutely very good.
It didn't flower this year. But I will observe if I get the problem of too many white boring flowers on this one.
If so, that suggests that the Japanese do have a special trick to make sure the environment causes more sporting.
The opposite problem of only solid red or purple flowers is white flowers with no, almost no, very little, boring, or all the same types of patterning.