Step 1- Line art on it’s own layer. (I used Denis Zilbers pencil brush see my post here on all types of great brushes.*see the end of post for screenshot of my layers)
Step 2 – Add a flat mid tone beneath the line art ( Use magic wand to select negative space with contiguous checked, than select inverse, than go to select/modify/contract by 2 pixels- than fill with desired color – this prevents space between fill and line)
Step 3 – Add a new monotone values layer above the mid tone layer defining the light source/s and forms ( I used 4 values for this picture the mid grey plus 2 darker and 1 light. Be sure to alt + click between the flat layer and the value layer above it to auto mask to the flat layer – now you don’t have to worry about going out of the lines)
Step 4 - Add a color gradient layer over the value layer to define the key color/s (Use the same alt + click trick as before. I used a complimentary color combo here. For a great Corel Painter like color wheel to add to photoshop see me post here)
Step 5 – Add some paint strokes for reflected lighting/rim lighting on another layer above ( Look beneath the nose and chin – I used a saturation layer – you can use a different color/source to make it more interesting)
Step 6 - Add a background beneath all the layers ( I used a gradient plus some interesting texture brushes see the links above for brushes and texture resources)Step 7 – Final step -Add an adjustment “levels” layer above all the layers to bring out the dark and lights of the picture and make it richer.
This method separates value painting from color painting. It is a classic approach and is used by many pro artist.
A popular alternative method is to paint the local colors first and than the values in color on top of them.
A middle ground might be to fill the canvas with the key color/s and than paint the middle/dark values and darks in a more or less “monotone” of the key colors and finally add more diverse colors and color accents toward the end along with the light/middle and highlights. I think this final method might be the fastest for many pictures while still making it easier to separate value from color.
One step that many digital artists add to this process is adding textures in the form of photos or brushes on an adjustment layer like overlay or soft light set to a mid or low level transparency.