The grey-green color is more visible in this photo as it reacts with the lime it first darkens then lightens as it dries.Lime plaster doesnt actually really dry,it undergoes a process in which the lime reacts with carbon dioxide in the air and hardens back into limestone.When pigments are dispersed in water and added to the lime while it is still wet,or open,the colors are then bound into the lime and become part of the surface.Unlike other forms of painting,such as oil or watercolor which form films on a surface,with fresco,colors are integrated and become part of the surface itself.This process plays a role in the unique way the color of fresco is reflected to the eye as well.There is a luminous quality to the colors in fresco because of the way light reflects on the lime,the quality of the color of white of lime putty is unique.It is different than the way color is seen in other types of paintings like oils,in which the white ground is reflected back through the various layers of color to the viewer.Fresco has a more chalky calcium white type of finish,somewhat like seashells.