I do as the others have described -- add a larger dose of FO to the portion of soap that has color. But that works only to a point -- there's a limit to how much FO the soap can hold.
If I would normally use an FO at 4% ppo in an entire batch, but I decide I want all of that FO in only 1/3 of the batter, the dosage in that portion of batter would be 3 X 4% = 12% total in that portion. This much FO is very likely to weep out of the soap and look unattractive or it can even form small, wet pockets within the soap.
Experience has shown me that soap can typically tolerate about 8% FO maximum without having problems. Maybe as much as 10%, but that is getting "iffy."
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I want to encourage people to let go of the idea of "water discounting." I know I'm being picky here, but this concept is so unhelpful. It doesn't help to explain the chemistry of how water, fats, and alkali inter-relate to make good soap. It also creates a mental barrier that prevents soap makers from seeing the water content in a soap recipe as a number that can be adjusted higher or lower to achieve specific goals.
If you look into it, you'll also find there's no one specific water content that is universally accepted as "full water." A number of definitions of full water have been espoused over the years by various soap making authors, bloggers, and forum posters. If there's no one specific water content defined as "full water" then how can you "discount" the water in any meaningful way?
"Full water" and "water discount" are concepts that many people accept at face value as being true, but they make utterly no sense when you look at them closely. I encourage people to talk about lye concentration or water:lye ratio simply in terms of the practical effects they have on soap.