Question

Why does Nitric acid turn skin yellow?

Answer 1

Aqua fortis , also referred to as nitric acid and spirit of niter, may be a highly corrosive mineral acid. The pure compound is colorless, but older samples tend to accumulate a yellow cast thanks to decomposition into oxides of nitrogen and water. Most commercially available aqua fortis feature a concentration of 68% in water.

Complete answer:
Nitric acid is understood to cause yellow stains when it comes in touch with human skin. This is often mainly thanks to it reacting with the protein called keratin that's found within the human skin. This reaction is usually termed as xanthoproteic reaction. During the reaction yellow colour nitrated products called xanthoproteic acid is made . Aqua fortis naturally is corrosive to the skin and may cause severe burns, scarring, ulceration, and will be handled carefully.
Nitric acid is employed for the assembly of nitrate , a serious component of fertilizers. It's also used for producing explosives like nitroglycerin and trinitrotoluene (TNT) and for oxidizing metals.
The natural manner during which nitrates reach the soil involves the reaction of nitrogen gas and oxygen gas within the atmosphere to make dioxide gas ($NO_2$), which then reacts with atmospheric water, making aqua fortis , which provides a natural source of nitrates in water and soil.
Commercial aqua fortis is yellow in colour due to presence of dioxide but when it's diluted with water, it turns colourless because dioxide gets dissolved in water.

Note:
This dioxide when it comes in touch with acid appears yellow in color. This suggests the colour of aqua fortis are often removed by bubbling air through acid.
Aqua fortis ($HNO_3$) may be a highly corrosive liquid that's toxic and may cause severe burns.