To create this Marker Pen effect, use a relatively large size brush for the area you are colouring in and take the hardness of the brush down to about 70% and opacity to about 60%. The line drawing will ideally have a transparent background and be kept as the top layer throughout. Colour each shade on a seperate layer and erase the excess. To perfect the shading use the layer opacity controls and also layer effects (colour overlay) to change colours completely. Marker Pen sketching is a quick art so the best sketches will always have areas that are not perfect with some shading missing or crossing the lines.