When I start making a character concept art I usually do some quick silhouette thumbnails and pick the one I like for further elaboration. Such quick sketches really help to discover some new ideas and you never know what will come of it in the end. This is quite exciting process:
After picking a silhouette I start to look for some references. I always try to have some references in front of me while working on the concept. It helps to push imagination further and suggests some new ideas. But the important thing is not to copy your references blindly but to use them as a direction in your work. This way you will learn a lot.
The next step is making a sketch of the character`s pose. I always start in black and white to determine gesture and silhouette. Too much detalization at this stage is a bad idea because I know that I will have to over paint it entirely when it comes to adding colors.
When I`m happy with the pose I proceed to colors. I use three ways of putting colors on a black and white picture:
1. Simply putting colors on top of the black and white sketch! (My favorite method)
2. Using an Overlay or Color layers over your picture to find out what you want.
3. Using Color Balance or Curves (experiment with the channels) adjustments on your picture. These are very handy tools.
After the color appears I started to define my picture.
When I work I try to put all new or questionable ideas on individual layers. This helps to keep your work clean. It also gives you an easy way to select parts of your picture and adjust them separate from the other stuff. For example the mask on his face was added at the last moment (I knew I wanted it to be there but wasn`t sure it would fit):