Just a simple outline of the steps I took to do this

This is only my second time coloring in this method, so don't feel like you'd have to adhere to it completely.

Step 1
First I make a basic body outline on a new layer above the background. Since I have not yet designed the armor, the body provides the guidelines in the next step for how the armor will look. I generally use a 4-5 pixel wide brush with a light gray color in the sketch.

Step 2
Make a new layer above the body. Lighten the body layer, and with a darker gray brush (and probably a little thinner, say, 3 pixels) start designing the armor and details as you wish. Look up refs or whatever in this step to help you out. When you're done with the armor/outfit/details, erase what parts of the body layer you don't want, then merge the two together.

Step 3
Duplicate the body/armor layer. Lock the pixels of the bottom of the two layers and fill it all in with a medium gray using a large brush. Then, unlock it and fill in the rest of the body with gray. Set the top "line" layer to "multiply."

Step 4
On the all-gray layer, start shading with shadows and highlights. It's good to draw indicators of where your light sources are coming from to keep them consistent. More light sources can often help to bring a picture out.

Step 5
Set your shading layer to multiply as well. Duplicate this layer and put the new layer below the others (right above Background), then lock the pixels. This will be your color layer. Use whatever colors you want to add color to the image; the shading portion is already done, so it will mostly consist of flats. However, it often helps to add different gradients for the shadows and highlights, for example, a less saturated, blue-like purple for the shadows on the armor and skin, with warmer hues for some of the reflections for the second light source. Finally, make new layers above and add new details (for instance in this picture, sharpening some rough lines and adding a soft glow to the armor) as necessary.

and VOILA! You're done! Hope this helped!