Aquaregia is a mixture of conc.HCl and conc.HNO3 in 3:1 ratio When gold dissolves in aquaregia, the species formed is AuCl4− in which gold is in +3 oxidation state. In the absence of air the reaction between gold and aquaregia
Au+HNO3+4HCl→H++[AuCl4]−+2H2O+NO
NO2 will not be produced. Yellow colour is due to the presence of NOCl and Cl2