I would color with a dark back ground (like a dark grey) and then figure out where you want the light to hit the bot, then color. Big brushes first then work your way down to small brushes, that'll be your detail. You might've already known this, but if not I hope this helps.
It looks nice with first glance. After two, three glances later, I have a few things to pick on:
composition wise, right now you cut the robot left hand off. When I saw the image, I was waiting for it to load more. When I realize that's the full image, it feels like you cut him off in a bad spot. I think it'd go better if you extend the bottom more, have more of the left hand in the picture, maybe only a tip off the border.
It's cool to see simple silhoutte against simple background, but in order to have it more polished looking, I'd go touch up the edge to avoid blur and jigged round spots. Since this is a WIP, those spots probably won't be a big deal as you work more on it.
Thanks for all the advice everyone, this is the progress i have made from last time, some planning lines in there and a bit of colour has gone on. The background colour is just so that i can see what im doing easier.
Like look said, I think you should extend the bottom of the pic a bit more to show the hand of the robot. I think the bundle of wires connecting to the left arm of the robot kind of throws the image off balance (unless you plan to do that with the other arm). I also think the whole design would look better balanced if you re-added those cylinders on it's left arm.
Right this is as close to finished, i resolved some of the design issues as advised but could be bother to add on another section. This is as far as i can go with it at the moment without smashing my tablet to bits.