Attainmenet of "equilibrium state" with the help of "constancy in intensity of colour" is noticed in the case of following reaction occuring in a closed vessel

Iodine vapours are violet coloured. Constancy in the colour observed indictates equilibrium state.In the a) Decomposition of CaCO3          b) reaction of N2 and O2          c) Decomposition of  NH3none of the reactant (or) product is coloured.