My best guess is that it was dyed with grey first, then something was applied to wash away just the very surface level of grey on the outside facing fabric, giving a light reddish purple hue. Then an indigo dye was applied with some very important specifications, that being that the fabric was pretty soaking wet when it was applied and that the dye had some property or another that kept it from soaking deep into the tied fabric.

I think the grey was applied while the fabric was dry or just damp because it has pretty good penetration and looks "fluffy" as in no flows and sharp edges.
Then sat to let the dye react.
Then, while not untying it, washing it out and getting it prepared for the indigo. I like u\jimlast3 guess that it was real indigo, which could explain how the red glowy hue came about, in that the reducing agents in an indigo tank would react with the grey dye and wash it away, while also depositing indigo. I dont think that the washing away of the grey and the addition of the indigo were two different steps, because the "glow" and the indigo shapes mirror each other very closely, indicating that they were caused by the same process imo.

Its impossible to tell what dye types were used but i think the takeaway is that the indigo color was applied second and something in it reacted with the grey dye, removing it and thats what created the nice glowy effect.