[antonio]

is there any rule in order to know how will be the color of a precipitate?

i really want to know it, if there's one

[kyle1990]

There's not really a rule, but the general statement is this: Compounds that have elements containing transition metals will be colored (such as iron, copper, chromium, etc...). This is because of the splitting of the d orbitals (some have lower energies than others, causing the electrons to jump around). This is very general,  but it's a good rule of thumb. Zinc is one of the exceptions since all it's d orbitals are filled. If you want to know more, look into the crystal field theory.
hey