Elements are different because their atoms contain different numbers of

This is the correct equilibrium represented in the question

Blue + 4 Br- (aq) <---> green + 6H2O (l)

Addition of NaBr supplied the Br- ions which moved the equilibrium position towards the right hand side. The forward reaction is endothermic hence its rate decreases when the reaction mixture is placed in an ice bath.

Explanation:

Blue + 4 Br- (aq) <---> green + 6H2O (l)

Looking at the equilibrium represented above, addition of Br- moves the equilibrium towards the right hand side, that is, the green colour. Addition of NaBr increases the Br- concentration and drives the forward reaction hence the equilibrium position shifts towards the right hand side. The fact that the solution turns blue when placed in ice bath reveals that the forward reaction is endothermic. Decreasing the temperature shifts the equilibrium position towards the left hence the resurgence of the blue colour.