This is some quick painting tutorial for me, I use Aobe PS and tablet.

1. First, U have to make a sketch of ur painting. Use random brush and make a quick scratch with it. For me, I prefer to use default hard brush.
2. Now W e are going to make this sketch enhanced and smooth. Blend all that gray color,but don't use smudge tool bcoz Ur PC will be working hard with it. So, I blend that with pick tool. I choose Chalk 36 as my brush and then pressing alt+click on the color i decided(this are the shortcut to pick a color). Now u have to scratch Ur brush around to make ur sketch smooth.
3. After the sketch are smooth enough, give some textures on all body parts. For the armor I choose linear dodge in the layer style option and decrease the brightness. Mask it till U get a natural looks of the armor.
4. After that, I make this paint more soft, u can use a softbrush dodge and burn tool or.. Just duplicate Ur layer, give a blur effect on the bottom layer, and choose overlay on layer style for upper layer.
5. Now We are going to coloring It. Choose a smooth brush on Ur Brush tool, Make a new layer with layer style named "color". Just brush on the grayscale pic with the color u decided.
6. A little paint over and, Ur paintings are finish.

* If u don't have a tablet, U should play with brush opacity and eraser tool to make a good sketch.
* Sorry if my hand type hurts U
Enjoy