Both statements I and II are individually true, and statement II is the correct explanation of statement I.

Statement I states that the color of nitrogen dioxide changes to colorless at low temperature. This is true because nitrogen dioxide is a reddish-brown gas at room temperature, but it undergoes a reversible reaction with itself to form dinitrogen tetroxide (N2O4) at lower temperatures. N2O4 is a colorless gas, explaining the change in color.

Statement II explains that at low temperature, nitrogen tetroxide (N2O4) is formed, which is colorless. This is also true and supports statement I. Nitrogen tetroxide, or N2O4, is formed through the reversible reaction of nitrogen dioxide, or NO2, at lower temperatures. N2O4 is indeed a colorless gas.

Therefore, both statements are true and statement II provides a correct explanation for the change in color observed in statement I.