Well, artgerm has a livestream for the survivor piece:
http://livestream.com/stanleyartgermlau/events/1777578
you have to scroll down a little, but it shows his colouring process for it. Looks like he starts with a dark flat layer and building up with a soft brush using muted tones to get the atmosphere of the picture right, and then slowly adding in more saturated colours so as not to use the darker feel. The software he's using doesn't look 100% familliar to me (looks kind of like photoshop but not) but I think you could use any software for it so long as you had a soft brush.
the 'Dota 2' piece, I imagine, uses a similar process but using more vibrant flat colours to start with. Both are common processes, but I think he really spends alot of time on rendering the shadows and lighting, even the subtle parts, and that's what makes his style distinctive imo ^^
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I could be wrong, but the greek warrior piece coloured by Allain Leon looks like it uses soft colouring, a couple hard brush highlights, and then a noise filter whacked on for some texture. I'd recommend pursuing some other ways of applying texture though, noise filters look quite uniform sometimes and a little unrealistic. But again, beauty is in the eye of the beholder and it could be a style you're aiming for.
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hmmm. Gonzo has some tutorials but not of this style which is kind of more like an illustration. I would say he used a hard round on the face and hands, and a textured brush on the robe and background. Might just be standard photoshop brushes. I'd say they're all set to high opacity and flow, there's nothing soft about this image. He may have had a 'black' layer on top set to multiply for the major areas of shadow to make sure he didn't lose them while colouring, but I'm just speculating at this point.
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but yeah, for the most part it'll be flat colours next and then building up the shadows/highlights. Having a couple references for the light source is always bundles of help for getting the people looking realistic. Also, if you're going for a comic book style, may I recommend Wolf Amoung Us sort of style for the guy?
it's dead simple: black for the shadows, a cold colour for the ambient light, and a warm colour for the main lightsource, and a nice texture on top.
Anyway, that's just some suggestions :3 Your lineart is already gorgeous and I really love the blue girl in particular xD
