I have come to the conclusion it is the one with the least amount of color to it. Right now, I'm trying Great Value "Light Tasting"Olive Oil. I've had EVOO and regular OO, but they make my colorants a bit funky. I used blue colorant and got a green soap. It's turning brown as it cures. The pomace oil is a chemical extraction process from what I've read. Don't know if that makes a difference in your choice.
I dont use EVOO because its too expensive and I save it for the kitchen, and its the greenest one. I like the plain Kirkwood or Pompeii OO from Cosco. Pomace is nice and cheap, but it does trace faster.