The reason behind this phenomenon is that the pigments used for coloring are very small particles and the distance between the particles determines how intense a color is. If the distance is big, the color is paler and lighter, if small, the color is deeper, more intense. When foaming, the plastic material is expanded and because of the increase in volume, these pigments are moving farther away from each other. In a foam with 30 kg/m3 density the distance is bigger than in a 100 kg/m3 density foam.