It takes time to dry.
One of the biggest difficulties of affresco technique is that the painter does not see the actual colors while working. Any pigment saturated with water changes color. It almost always darkens as well. Affresco colors are ground with lime and/ or water. As the color drys the value and color of the pigment return to its almost original state. Therefore, the artist’s palette in affresco is always pre-mixtures.
Colors are all premixed and tested before being used. Most of the flesh tints while wet are orangish and difficult to differentiate from each other. So the actual process of painting is broken down into definite steps: color 1, then color 2, etc. Most often it is going from light towards dark. However in the hands of any expert artist, rules are often broken and only experience and the specific needs of the project prevail.
The images above are of the 2 sketches I did of my brother Jon after drying for 4 days. You can see from my last post how much the colors have changed. The image below is of the third version that I did just before leaving to teach a summer course in Florence. The colors are still quite fresh in that one. Once I return in August, I will photograph the sketches again to show the changing colors.
It is always difficult to judge colors and values from photographs. The general tendency in affresco is that the colors lighten. Hopefully the photos show that.