>>5700609I learned it over a decade of studio/live model oil painting, I'm not really sure where to find documentation pertaining specifically to toys. It works with acrylics just as fine as oils - in more specific terms, the concept is called "glazing" and relies on the idea that layered color transparency allows light to pass through to lower layers and bring out the underlying colors in a visual mix. It's basically the same physical phenomenon that allows you to see the color of your veins through your skin (or, to make it more relevant to the discussion of toys, how a base coat of gloss black on Gunpla brings out silver paints even better, which in turn makes candy coats look amazing). When light can pass through an object, it will introduce the reflected (but dulled) colors of what is below.
Broadly speaking it's like unconsciously detecting green, blue, and red in the same area but consciously registering it as realistic flesh tone paint with your brain. It's one of the accidental marvels of the limitations of the human brain, where a computer would discern every last individual color uniquely and ruin the effect but the limited physical and cognitive capacity of humans makes everything kind of blur and look great.
In oils we use a liquid medium (like Liquin) to dilute the pigmentation of the paints and allow for it to be applied transparently as a glaze, and with acrylics essentially what you're doing is spreading pigments with water in an identical fashion. So on top of the stock flesh color paint, you would start to gradually pile up nearly unnoticeable transparent layers of, say, magenta to give the illusion of blood flow coursing through the "T" zone of the face (i.e. the area that bleeds really easily, from the nose to the bottom of the brow ridge).
>tl;dr: words words words, thin your paints even further 
