usable mineral colors do get ligther when painting al fresco, between 20% or 30% ligther during the following six months, due to the corrosive nature of lime, after that period, the pigments will remain very stable.
there are frescoes that are 3000 thousand years old!!
due to the live nature of true fresco many factors can also affect the result, mostly modern contamination (carbon monoxide is a killer), proper wall preparation (salts are a public enemy of fresco) and proper painting...
in general, even if fresco painting is very rare nowadays, we do now quite a bit about the things that work and the things that dont..