It started as a "color wheel" highlighting experiment when I painted
Rackham's Capt. Kelian Durak. This time I did it a little differently, since I wanted dark, brooding colors (listened to a lot of Lego Batman's "Darkness" song)....
Basecoated in Reaper Dark Elf Skin, mid-tone 50/50 mix Dark Elf Flesh & Vallejo Scarlet Red (awesome color for this effect
BTW), successive highlighting layers by adding more and more SR to the mix. Then a dark purple wash, then more highlight layers until the final one of pure SR.