Why the pale blue liquid of N2O3 loses its colour on warming?

Answer 1

N2O3 is a pale blue solid. Above its melting point of -100 0C it dissociates

N2O3 <-> NO + NO2

At low temperatures the liquid is quite an intense blue but fades to a greenish colur due to the presence of NO2.

Brown monomeric NO2 is in equilibrium with its dimeric form colourless N2O4

Answer 2

The pale blue liquid of N2O3 loses its color on warming due to the decomposition of N2O3 into NO2 and N2O4 gases, which are brown and colorless, respectively. This change in color is a result of the different electronic configurations of the two gases compared to the original N2O3 molecule.