Great costume idea, first of all! Here are some general tips:
1. Elongate your eyebrows - pencil them in, or pencil over your actual eyebrows. By elongating, and angling the eyebrows up it gives you a sexy, evil look and changes your entire attitude.
2. Smokey eyes are a great fall back if you don't fall in love with any other ideas. It's simple to do, and always looks great. You can also start with smokey eyes and add on as you see fit. Greens and Reds are colors you're going to want to include in your makeup!
3. Liquid eyeliner will be your best friend. You can draw very sharp or very thick lines. It looks great. You can have so much fun with it, and it's super easy to take off or change if you mess up! It also comes in great fun colors - such as green or gray.
Okay, enough of that... here are some photos to help ya out. Enjoy!
Instead of blues you could definitely use greens and blacks.
You can create that sort of scaly look by using a variety of stencils - i've found that fishnets actually work great for create different patterns with makeup - just cut a piece out and put it on your face and either paint or draw into the circles, or use an airbrush if you have access to some hardcore makeup tools!
And finally this is a good look if you want to keep it simple and easy. Liquid eyeliner and diamond appliques work well. You could take this look and do a lot with it, too - add some red or green light eyeshadow underneath and around your eyes to look like a light mask, kinda like this:
Have fun and good luck!