$\begin{array}{cc}F{e}^{3+}\left(aq\right)+SC{N}^{-}\left(aq\right)\rightleftharpoons & \left[Fe\right(SCN){]}^{2+}\\ \text{ Y ellow }& \text{ Red }\end{array}$ Addition of the following increases the intensity of red colour

Oxalic acid and HgCl₂ decreases the concentration of reactants and equilibrium favours backward reaction increases intensity of yellow colour

addition of KSCN increases the concentration of reactants and favours forward reaction increasing intensity of red colour