Coloured Pencil – good for shading and tone. Lends itself to hatching well and can combine colours eg working with hatching technique using different colours, using variety of light to darker colours, combining colours to achieve colour variation and depth of tone and impression of texture
Watercolour pencils/Aquatone – Just the same as coloured pencil with added benefit of being able to add water which will give a much smoother blended effect. Coloured pencils/watercolour pencils give a soft effect and so would be suited to the task if that is the effect you want!