Since I already decided upon the light source, now I can start to add blocks of colour and details, depending upon the direction of the light source, and can work out where the muscles, clothes, and arms are.
Here I want to explain why I paint the background first. It’s just like painting in watercolours, or in oil. In Photoshop, with 50-90% opacity of your brush, painting on the background, you will find it will help your character to immerge from the background. Here I separated 3 layers, one is skin, another is clothes, and the other is arms. It’s more convenient if I am not satisfied with the colour and want to change. I always use Burn and Dodge Tools to make shadows and highlights, playing with the “opposition”, checking the perspective and proportion. I used “Colour Balance” to apply the colour for the helmet and arms, and “Filter>Noise>Add noise” to make the bump.