I assume that these onions contain a pigment called anthocyanin which can also be found in red cabbage and beetroot. This assumption was proved correct with the resulting colour. The blue colour of anthocyanin mixed with the yellow pigment that also exists in regular onions provides a beautiful kharki. We have yet to try regular onions, but suspect that they will form the base of our next experiment.
The images below show both the boiling stage and finished results on a raw silk shawl.