Hint: The air we breathe is made up of a variety of gases and microscopic dust particles. It is the clear gas that all living beings breathe and live in. Nitrogen, oxygen, which is the life-sustaining material for animals and humans, carbon dioxide, water vapour, and trace quantities of other elements make up the atmosphere (argon, neon, etc.).
Complete answer: Light is needed for us to see an item. Some colours will be absorbed by an object after it has been struck by light, and others will be mirrored. The colours displayed by the item move to your eyeball, where the brain interprets them. Since our eyes can only see a small portion of the colour spectrum, all we see must be reflecting light that falls within that range. It points out that the colours that can be reflected by air do not fall within that visible spectrum. Our field of view would be continually disrupted by the air in front of us if humans were capable of seeing such specific reflections. Every day would be like living in the centre of a massive cloud; slight variations of colour would be apparent, but we wouldn't be able to see clearly due to the cloud.
Note: Since we have developed retinas that are open to the same wavelengths of light that travel into it unobstructed, air is invisible to our eyes; otherwise, we wouldn't be able to see anything at all. This isn't a coincidence; rather, it's a product of how the human eye has developed through time to guarantee that our vision isn't blurry by default.