Why does nitric acid not turn yellow everytime?
Pure nitric acid is colourless, the brown colour is caused by dissolved nitrogen dioxide, $\ce{NO2}$. 

Nitrogen dioxide is formed by the slow decomposition of nitric acid, according to $$\ce{4 HNO3 -&gt; 4 NO2 + 2 H2O + O2}$$

Nitrogen dioxide is a good oxidant and it does so - supposed that it finds a suitable partner.

This would be chloride. So, the more chloride in solution the more redox chemistry furnishing colourless products.