The Color Modulation set from Mr Hobby is a four colour set of weathering colours for adding depth and realism to your model. Varying shades of darker and lighter tonal colours add dimension to the model in order to highlight it's features.

The actual figure will look three dimensional because of the shading as light falls on it, but if nothing is done after painting, the figure can give a sort of flat impression, has little presence and becomes dreary. The technique used to emphasize the stereoscopic effect by changing the tone of even just one color is called "Color Modulation". In some cases where you have a small piece such as a model, it is not easy to see the change just by changing the color of each surface. So gradation painting is used to emphasize the stereoscopic effect.