Brown colour in $$HN{O_3}$$  can be removed by

Solution :
Nitric acid decomposes to give $$N{O_2}$$  which is brown
$$4HN{O_3} \to 4N{O_2} + 2{H_2}O + {O_2}$$
When air is passed through acid the reaction proceeds towards left hand side and brown colour diminishes.