You did alright. The face is cute, (I totally imagine she'd be tsundere. lol) and I don't have any issues with the design as far as content goes. Her human arm however, is uneven in length, and your coloring could be improved by some quick tips.
If you're using Photoshop or something similar and you want to blend colors to shade, set your opacity between 25 and 30 with the paint brush, and make a number of strokes until you have the desired darker color. As you make successive strokes, you'll see the colors start lighter and gradually get darker as they overlap. By holding "Alt" you can bring up the dropper tool and select one of the lighter shades, and then blend that together with the darker one. Do it every couple of strokes and you can get a gradient effect like
this That's your basic technique for shading using opacity in photoshop and similar programs. BUT You should probably not use this technique for your robot arm. Highlights and shading on metal are different and more intense than on other materials. Try googling some metal objects to get an idea of how the reflection works, and then try to apply that to how you're coloring that arm.
Aside from that, I'll just say you should push your values more. There are a bunch of crazy things to learn about coloring, which Lenorekitty brought up (such as reflective light, and using contrasting colors) but focus on technique for now, because once you have that down it will be easier to apply the more abstract concepts.
P.S. If you'd rather go for a cell shading style like that seen in most anime, try not to use zig-zags like the ones going across her chest unless that's part of the clothing. You're better off making it more of a sharp geometric shape, than an irregular one. And also, if you have a tablet and PS, I can show you how to set your brush to automatically adjust opacity to shade like I explained. The method I wrote earlier was more of a mouse-style.
Hope that helped some. ^_^ I do like the character.