Concentrated nitric acid, upon long standing, turns yellow-brown due to the formation of

Nitric acid decomposes in sunlight as per reaction
$4{\text{HNO}}_3⟶4{\text{NO}}_2+2{\text{H}}_2\text{O}+{\text{O}}_2\left(\text{g}\right)$
Yellow colour is due to dissolved NO₂(liq.)

NO₂(g) is brown in colour but it is yellow in dissolved form.