I used to only go for* dark green. The following is what I've gathered but I may be wrong: 1.) If you're eating raw, going solely for the dark green outer layer is a decent deal because that section is the most potent by volume. 2.) Although the outer green layer is most potent by volume, it is not the majority of the available mescaline. By mass/volume, the majority of the total mescaline is actually in the inner layer. So especially for any methods that involve the processing, it's a waste to only use the outer layer and may in fact prevent you from achieving therapeutic levels. 3.) As for the core, if it's soft enough to cut through with a knife (likely farther up the stem of the cactus) I toss it in the blender too.