Demon eyes have a tendency to have a lot of contrast. They're bright or blood red with few other colors. As you paint the eyes and choose darker colors as you shade, choose shades the lean more towards a brown than a black. This favoring with brown will help it keep a hint of natrual-ness and less demonic and otherworldy.
As for shading, it's hard to give advice when I don't know what program you are using or what coloring style you're going for. When I color, I use Photoshop and my technique's pretty simple. I put down a mid tone, pick a slightly darker shade that leans towards the color of my lighting, then I block in the shadows. I choose another shade darker and repeat the same process, but with a small brush and covering a smaller section.
Then, with the smudge tool set at around 30-ish pressure, I smudge the colors, blending them and guiding it to make my own custom sort of gradient. It takes practice and is easier to do it you imagine moving it like you would if you were paining with acrylics. Once you have your midtones and shadows in, do the same process but with a tone a little lighter over where your highlights will be. You just keep doing this till all your details are in.
I'm using MSpaint to do this. As for the colouring style, I'm not to sure. I just want a somewhat naturalish shading. When I look at the shading, something seems off but I can't quite place my finger on it. >_>
Well the only thing i can say about fixing the drawing is that the head seems a bit odd... the jawline seems a bit uneven and the hair starts too high, eyes are the centre point of the head so remember that if fixing it,however hair tends to stick up a bit so the eyes don't have to be completely centre. If your colouring the eyes red then remember that eyes have a:
pupil: the black part in the middle
iris: the coloured part, in your case red
sclera: the white part.
I tend not to draw humans with red eyes but if you are going for a realism just remember that red eyes only naturally occur in albinos due to lack of pigmentation.
Seeing as your colouring in paint I would highly recommend a cell shaded approach. i tend to cell shade. cell shading means not blending colours and thus,using a limited colour pallet in whatever your shading. but that doesn't mean you should just stick to only two colours.
I just remembered a quick sketch i did i think a month ago http://aeonzen141.deviantart.com/gallery/?catpath=scraps#/d4u6sz9 if you look at the skin you can see as the tones got darker i went from a very light orange to a grayish purple colour. I recommend you look at the colour wheel to understand why i did that. Remember this is only a quick sketch so i didn't put much effort or time into it (i think i spent around 13mins) so it's messy.
If you need help on anatomy or colours, look at tutorials.