The Delicate Dance of Woad's Chemistry
All Indigos including Woad, Indian Indigo, Tyrian Purple of the Murex snail and numerous other lesser quality plant sourced indigo, require the same complex balance of chemistry to occur in order to dye. The blue pigment indigotin, which is extracted from these different sources, is an insoluble substance that is not able to be dissolved in water. Therefore this indigotin needs to be converted into a form which can be dissolved to enable it to work as a dye.
The Tyrian Purple dye from the Murex snail needs an extra step in order to give a blue colour, rather than it's original purple. It's indigo molecule has 2 bromine atoms attached, red pigment causing it to be purple, that the plant based indigo molecules don't have.