I use 1 tsp ppo of activated charcoal mixed with 1/32 tsp. (a smidge amount if you have those pinch/dash/smidge measuring spoons) as my go-to black in my soaps. I purposely use white washcloths in my shower so I can test out my colored soaps, and my charcoal soaps using this amount ppo gives me white suds, but they do color my washcloths a very faint grey when showering..... which rinses right out under the shower head, easy peasy. It does not leave any stains behind.
This is the level of black that 1 tsp charcoal mixed with 1/32 tsp black oxide ppo gives my soaps:
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IrishLass