Painting Faces Step by Step

That are a lot of different approaches to painting faces, this is one that has worked for me.  The following example is a 54mm face (for Pegaso's French Dragoon) but I follow the same general idea when painting 28mm, 75mm, and 90mm faces.  The main difference is the level of detail I can reasonably expect to achieve.  But more on that later.

The following numbered steps correspond to each row in the image series seen below.  Unless otherwise specified the paint names refer to Reaper Master Series paints.

1 - The face is base coated with Rosy Shadow

2 - I rough in the shadows with mostly Chestnut Brown (I like to use a reddish brown for the shadows) but then a bit of Mahogany Brown in the darker shadows around the eyes, in the nostrils, under the chin, and a bit in the left cheek (his head is slightly tilted so the left side will be darker). The main reason I do this is to get the shadows around the eyes finished before I go in there to do those fine details.
3 - Start on the eyes
I first lay down a reddish pink layer (Rosy Skin + Violet Red) and cover up most of it with an off white (Weathered Stone). Ideally a little of the pink will remain in the corners.
4 - Eye details
I went in with a dark blue (Ritterlich Blue) and tried to get the shape of the irises. Because his head is slightly turned to his right I also painted the eyes looking a bit to the right. Straight ahead would have been fine, but to the left would have looked awkward. I then went in with a light blue (Ashen Blue) and tried to lighten up the irises near the bottom half, while retaining the dark border. Then, as carefully as I can, I add a black dot for the pupils and a dot of pure white for the catch light. Pretty tricky to do at the 54mm scale, so if you left the eyes at the dark blue stage you'd probably be fine. But if you can get those extra details that's great. On a larger scale, like 75mm or 90mm, I'd really try to get those in there.
5 - Returning to the skin, I go back with various mixes of Chestnut Brown and Rosy Shadow to smooth out the shadows I'd roughed in.
6 - Now I add in the highlights, with blends of Rosy Shadow into Fair Skin, and then into Fair Highlight.
7 - Details and Tweaks
Based on the previous picture I thought the highlights needed a little more pop, so I went from Fair Highlight to Linen White on the tops of the cheeks, tip of the nose, and a touch on the chin. I use a mix of skin tones and red for the lips, then repeat steps 5 and 6 for the ears. I also add in the eye brows with a dark brown.
8 - Glazing
The real magic comes with the glazes. I take some of the GW glazes: Bloodletter Red, Guilliman Blue, and a mix of the two for purple (although further thinned down with water). I add some red to the cheeks (pushing away from the highlights into the shadows), tip of the nose, and ear lobes. The blue goes on the lower part of the face to change the tone for a subtle stubble look. And the purple is used to deepen the shadows in the cheeks, under the chin and jaw, and a bit around the eyes. Lastly I go in a touch up a few of the highlights as needed.

Here is an example on a 90mm scale face.  The first image is the face after stage 5, with the eyes finished and the shadows blended in.  The middle image is during stage 7.  I'm added the details and am in the process of making a few tweaks (the corners of the mouth still need fixing, ears aren't finished).  The final image is after the glazing.

The main difference is the level of detail.  I put more work into the eyes, even doing some blending in the iris color.  You'll also notice a bit more detail under the eyes and I've created some subtle shadows to add in some forehead creases.  Also on this figure I used a mix of the regular skin tones and dusky skin to create the stubble instead of the blue glaze.

Some other tips and thoughts...

When painting around the eyes it's helpful to paint a dark line just above the eye (bottom edge of the upper eyelid) and paint a light line just below the eye (top edge of the lower eyelid).

On faces it can be very important to vary the intensity of your shadows and highlights.  The creases in the forehead are not very deep, so they get very subtle shadows (actually defined more through highlight than shadow).  The lines running from the nostrils to the corners of the mouth are deeper, but not as dark as the areas under the chin or right beneath the brow.  They also tend to fade the further they get from the nose.  I try to continually balance the shadows and highlights as I work on the face.  If some shadow gets too dark then just lighten it up.  Or vice versa.  As you go remember to take a step back and take a critical look at your work.  For example, on the Roman at the middle stage the corners of the mouth just didn't look right.  You don't always have to follow the sculpt.  You can choose to overemphasize or underemphasize features.

With larger scale faces I will sometimes work on the face in several sections.  I might just work around the eyes and nose, then the cheeks, forehead, and finally mouth/jaw.  It's not something I do every time, but sometimes I find this approach helpful.