Hi Maria thank you for posting!
I would like to add that painting on plaster is different from coloring the plaster itself and for painting sand "absorbs" pigments better then marble meal/dust because plaster made with sand is naturally more absorbent since the water circulates through easier hence pulling the pigments in much stronger.
It is easier to judge the final color with pure white plaster (made with marble sand or dust) since it wont lighten as much as the one made of sand. This is perhaps the reason you had "fading" usually doing your mix 2 values down (1-2 shades darker) solves the problem.
fine sand or marble meal and dust plasters do give more polished finish however it is at the expense of faster "locking" - stropping accepting paint. It is more difficult to paint on plaster made with fine sand or marble dust. Also to really take advantage of the white-marble-like surface the one should be very precise or it will loose transparency and will look just the same as if it was painted on regular fresco plaster.
Hence at the beginning until experience and advanced understanding of actual fresco painting is gained it is wise to start working with 1part lime to 2parts river sand or a classic mix of - 5parts lime putty to 8parts of river sand (Sistine Chapel Ceiling is done on it).
PS. in 12 years painting frescoes it amounts to about 10% of my work.
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center