faces are a bugger, there's no way around it.
eyes, I tend to do as follows:
1) paint all of the rest of your face. shade, highlight etc.
using a mix of black ink and black paint (so it is low viscosity but still intense colour) and a small teeny brush, paint the entire eyeball (but not eyelids or cheekbones) black. Make sure the wee crevice between the eye and the lid is black.
2) Using thinned white paint, paint the eyeball white. It may take 2 coats. If so, fine. Make sure the aforementioned crevice stays black.
3) Paint a small teeny line of black in the middle of the eye make sure it goes from top to bottom. There should be no white above or below it.
4 ) This is crucial: make sure that both eyes are looking in the same direction. You'll probably need to repeat the previous steps a few times to achieve this. I know I do.
You can change the eyes' expression by how you paint the upper eyelid. Eyeshadow works well for elves (i.e. paint it a colour, not skin colour. You can extend it out in a point right past the eyebrows for an especially exotic appearance.)
If you want a brooding look, paint the eye socket darker. For a vamp or something you could use a dark red-tinted version of your skin tone. For an emo elf you could use a bit of dark purple plus your base skin.
For actual skin tone, for eldar (or elves) I tend to use bleached bone (or equivalent pale flesh colour) as my base. I do the whole face with this, then add a little terracotta to it to make a shade for my Craftworld Eldar, and use a thin version of this to shade with. I probably water it down about 2:1 (water:paint). I put this mix on the underside of the jaw, under the nose, in the eye sockets, between the brows, behind the ears, below the lower lip, and (with an upwards brush movement) on the cheeks, aiming for a darker tone just below the cheekbones. I then add a smidge more terracotta and go back and do everything again, except the cheeks.
For highlights, I add skull white to the bleached bone, and do the bridge of the nose, the forehead, the chin, the cheekbones and the ears. I'll use straight skull white on the tip of the nose and the ears. This will give you a damn pale elf, though.
For my dark eldar, I do the same thing but start with a wierd pale lilac colour I mixed ages ago, and add purple ink/white for shade/highlight.
Finally, for elves I find painting the lips helps a lot. For dark eldar I use black, purple highlight, for craftworld I use terracotta, terra+beige highlight. Go easy on this and only do the bottom lip. Unless you want them looking like theyre wearing lippy, try to keep the colours fairly close to a darker version of your skin tone.
hope that helps.
*edit* You can kinda see the eyeshadow thing I was talking about here:
http://i84.photobucket.com/albums/k20/m ... /capn1.jpg
http://i84.photobucket.com/albums/k20/m ... quezbg.jpg
Note that their eyes are just straight black or white. Meh, so I'm lazy. The photo washed out the shades something awful. I really must take proper pics of my dark eldar soon and get my Hobby Log updated.
eldar
http://i84.photobucket.com/albums/k20/m ... rdians.jpg
http://i84.photobucket.com/albums/k20/m ... greyBG.jpg
these are cryx ladies. They are actually a good deal more complex, with pale purple highlights and dark green shades in the skin. You can play with colours. As long as you add a darker colour to shade your base skin and a lighter colour to highlight it, anything is fine as long as it doesnt overpower the base tone. Thinning and not using straight colours is key. (e.g. for purple shades dont use purple. use purple + base skin tone.)
*edit, again* sorry - those pics aint very helpful. I've better camera-fu now, so I'll try and get better pics soemtime later today. I have a shedload of essays to check first, though, so dont hold your breath...