I suspect it has to do with the particular quality of his silence.

In this case, purple refers to a particular quality of marijuana.

Because it has a particular quality which no other has?

There was a particular quality in the eyes of the witnesses.

He was no longer so great because his particular qualities were not needed.

There was a particular quality to his voice as he asked, "What is your plan, Commander?"

Only quoting can give an idea of its particular quality, and yet it is terribly hard to quote.

You needed very particular qualities, or lack of them, to be able to kill in cold blood.

I felt it needed someone with particular qualities and expertise.

But lumped in here along with so much else, they lose their particular quality.