When nitric acid is left for long standing, its color changes due to decomposition reaction.
The reaction in which one reactant yields two or more products, it is called a decomposition reaction. Due to decomposition, concentrated nitric acid ($\mathrm{H}\mathrm{N}{\mathrm{O}}_3$) gets broken down to nitrogen dioxide, oxygen, and water. Due to the formation of nitrogen dioxide, it turns yellow-brown. The chemical reaction involved in the reaction is as follows:
$\text{4}{\text{HNO}}_{\text{3}}\to \text{4}{\text{NO}}_{\text{2}}\text{+}{\text{O}}_{\text{2}}\text{+}\text{2}{\text{H}}_{\text{2}}\text{O}$