There is a chart that follows this page that will help you out some, but for me , because I have done it so long it just comes as second nature. Once I have the face sculpted (and most usually after the doll is costumed and the hair is put on) I just begin to paint on the face. I always begin with the eyes. They dictate to me pretty much how the rest of the face will be colored in, and the shape of the mouth.
I begin with a white football shape where the eye will be. I extend the color up into the lid area. And as often as not, I use an off white, especially on the darker skinned dolls. Misty white works best for the fairer skinned dolls. I then take my lid color, flesh tone, tans, or shadow color, load it onto the same brush and brush the lid in while the lid is wet, instant shading! I always go back and define the shading after the eye is painted in. Next I make a circle of my iris color inside the lidded eye. Again I work wet for highlights. Then I add a pupil, and remember when adding a pupil, that the larger the pupil the more "interested" and "alive the eyes look. After the pupil has been painted in, using the same brush, still with black on it, I mix a bit of the iris color with the black on my pallet to make a darker shade of the iris color and make a small outline of a ( ) parenthesis around the sides of the iris to add dimension. Now, I was my brush! I pick up a white and add a highlight to the pupil...remember to always add the highlight to the same side of both eyes to avoid the wall eyed look. Lastly to complete the eye, I take a very fine liner brush and line the top of the lid with black or dark brown, depending again on the skin shade, staying very close to the bottom of the lid. Under the eye, I line only to the eyeball and stop, then pick the line up on the other side and extend it to the edge of the eye...this leaves the eye open, and makes it look bigger. Using a color that is close to they hair color I am going to use, I then add an eyebrow. Now, I know that it is easier to paint on one eye brow, turn the doll upside down and paint on the other eye brow, but I don't usually do this. Attribute it to forgetfullness, impatience, or whatever you like, but I just go for the second brow, you know I like to be surprised by who this doll is and what she looks like, even at this stage.
Once they eyes are painted in, I go to the mouth. I usually paint in the mouth with a lipstick color, or a darker flesh, depending on the period of the doll, and the intensity of the color of the mouth I want. For darker skinned dolls, bronzes, brick reds, and maroons work best. I begin with a heart shape in the center, and draw out the sides of the mouth with my brush. Then I highlight the mouth with lighter shades on the bottom lip, and line the mouth if necessary with a darker shade of the same color, usually adding browns to darken the line. A bit of flesh smeared into one side of the mouth and mixed with the lip color adds highlight too.
I blush the cheeks with the same color, or one shade darker than the flesh tone, melon color works great! I do this by taking a flat angled scrubber brush, don't scrimp here, buy a good soft one, I load the brush with a bit of color and the use a paper towel to remove most of it, by scrubbing the paint off. Then I blush my doll's cheeks in the same way I blush mine with my make up brush...the key is removing most of the paint, and lightly scrubbing the paint into the fabric.
Then I stand back, add more shadow and shading to the eye if needed. Enchace the lips etc...but don't do too much, you can ruin a face by adding too much to cover percieved imperfections in your painting....step away wait awhile before you paint more. If you are still not satisfied in an hour, or a day, then enhance the face.