Question

why does addition of chlorine to KI solution gives it dark purple colour but excess chlorine makes it colourless ?

Answer 1

Cl₂ being a stronger oxidising agent than I₂, first oxidises KI to I₂ which imparts brown colour to the solution.
2KI (aq) + Cl₂ (g) $\to$2KCl (aq) + I₂
But when Cl₂ is passed in excess, the I₂ so formed gets further oxidised to HIO₃ (colourless)
5Cl₂ (g) + I₂ + 6H₂O $\to$ 10HCl + 2HIO₃