To distinguish between neurons and glia, Herculano-
Houzel injected the vials with a chemical dye that
would make all nuclei fluoresce blue under ultraviolet
light, and then with another dye to make the nuclei of
neurons glow red.
You mean that part? It’s about differentiating support infrastructure from the actual cellular operators responsible for cognitive brain function.
https://en.wikipedia.org/wiki/Neuroglia
https://en.wikipedia.org/wiki/Neuron