How is the yellow colour of nitric acid removed?
I think you can explain this using the equilibrium:

$\ce{NO_2(g) &lt;=&gt; NO_2(dissolved)}$

When you pass $CO_2$ through the warm acid, the partial pressure of $CO_2$ ($P_{CO_2}$) above the gas increases (Since partial pressure generally represents the amount of gas above the solution). 

Since


total pressure $P = P_{NO_2} + P_{CO_2}$ (Dalton's Law)
And $P$ remains constant at atmospheric pressure (assuming that the system is not closed).


$P_{NO_2}$ will decrease.

According to Le Chatelier's Principle, the equilibrium proceeds backward to compensate the decrease in $P_{NO_2}$ and thus more $NO_2$ gas is released. The gas flows quickly along with the bubbled gas and this prevents the forward reaction. 

This seems like quite a roundabout method but it works. I'm sure there could be a much simpler explanation though.

The oxidation mentioned is most probably the disproportion reaction of $NO_2$ when it comes in contact with water:

$\ce{3NO_2 + H_2O -&gt; 2HNO_3 + NO}$