I doubt it is by design. Usually it is by botrytis, a fungal pathogen that puts little black or grey spots on your white flowers. It may put spots on other color flowers too, but you see them best on the white ones.
I see the most botrytis when it is hot and humid in the greenhouse. Like the last couple weeks. I cut several dozen spikes last weekend that had nice flowers except for the spots, can't do much with them.
Keep your air movement high (increase it, if you have little air movement). You could try spraying with a fungicide before the spots appear (I don't use many fungicides myself or I'd give you a recommendation). Unfortunately once you have the spots, you may as well cut the flowers off because they won't get better.
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