This is a really important step, that will determine the mood of lighting as well as the direction I'll take in the final stages. The most important part is to view the shadows over the flats (as shown below) to get an idea of the three-dimensionality of the image as well as the feel of lighting with an absence of light.
Also, at this point, the coloring really gets fun. Shadows and flats are done, so we have a Mignola-esque image that completes some of the more difficult and main focuses of coloring. Everything after this is icing on the cake. Even some of the more difficult effects will be fun since, regardless of how many mistakes are made because of it, hey, the hard stuff is done! :-0
The next step will be coloring the shadows based on the flats. This is a new technique I came up with recently and the results are amazing. It really gives off the range from anime cel-shade. It's kinda tricky to understand, not to mention you want to have a good eye for color, otherwise the results can be disastrous. The good thing is that it's MUCH better than the "new layer in black at 40-45% opacity" technique that is way overused. Mainly because the effect leaves the colors looking a bit muddy. Color is a privilege, so why not take advantage of it?
Here are a couple of initial observation tips:
The lighting is somewhat dark and warm, reddish hues and such. Also, away from the blast we'll contrast with cooler colors to give the feel of night erupting. Electricity will flow slightly from the injured bot. Blue is always nice and cool. But we'll make it subtle. We'll also need a big glare of light from the blast.