Wow, Thank you!
The yarn analogy was really helpful for me! Thanks for taking the time to write a huge comment.
I did not consider this a WIP, but you are very right, it is in fact very WIP. I will start using the forum to my advantage! Thanks, Stitchy!
Hmm, this is definitely a good start with a small palette! But I'm gonna give you some analysis of how you could improve this pic. ^^
The shading on the hair is incomplete. The breaks between the hair strands are shaded, but the full form of the skull is not shaded. Think of the head sort of like a ball of yarn, each string is going to cast its own small shadow, but the ball will also cast a shadow over the whole of itself. Since the lightsource is coming from the right side of the image, the left side of the head should have a darker shade all over, and this also applies to the ponytail. To show you what I mean, take a look at this: http://i57.tinypic.com/r6wto1.png I isolated the two brightest shades (the "highlights") found on the left and right sides of the ball. The lightest shade on the left side is signficantly darker than the lightest shade of the right side. All of the shading on the strings on the left side is darker than the shading on the right side. Also, keep in mind that objects also can create shadows on other objects depending on the lighting, so the head is going to cast a shadow on the ponytail around where it meets the skull because the head will cast a shadow.
I find it best to begin with the shade of the general form first, and then add the details in from there. This isn't perfect by ANY means since I don't have a reference of your face, but here is some of the basic form shading I'm talking about applied to the image: i61.tinypic.com/25fork9.png Shading follows the form of the object, which is 3D, and not the outline, which is 2D. Your chin casts a shadow across your neck, and your ear is going to appear overall darker than most of the face.
This also connects with the fact that most of the facial features don't have much definition. You can barely see the form of your lips because the two shades you used are very close to each other, and the lips get lost. I would consider the shading on the neck and forehead to be a shade too dark; the lighest shade for the hair would be a better choice. Overall, I feel like the colors you chose are either too close to the next, or too far from the next. Color tweaking is tricky and will take time for you to learn, however, and I think I have said enough for this picture.
Sorry for the huge comment, but perhaps it might be wise for you to post your WIPs in the forums if you haven't already. You can get a lot of tips before you submit a piece and learn that way. :D
Hey, glad I could help! That's what fellow artists do, right? ^^ Keep practicing and observing and you'll improve and learn tons of new things~