Nitrogen dioxide is a brown gas. This is a sample of nitrogen dioxide in a completely sealed glass apparatus at approximately normal pressure.

The intensity of the color of the gas depends on the temperature. The higher the temperature, the darker the gas is looking. This is due to the equilibrium, which shifts to the right at lower temperature.

   2NO₂ ↔ N₂O₄

The compound N₂O₄ is a colorless gas. When this glass apparatus with NO₂ is put in a fridge, then the gas in it becomes completely colorless. When it is taken out, then it soon darkens again. At the temperature in a common fridge (-18 ºC or so), virtually all NO₂ is converted to N₂O₄.