Why is nitric acid yellow

I have dilutions of different samples, which are made with 0. Only I left them on the lab table for a few days and one turned yellow. So my question is why would one turn yellow and not the other ones? They all have the same concentration of sample. The matrix in all the samples are the same. This would be chloride. So, the more chloride in solution the more redox chemistry furnishing colourless products.

Nitric acid takes yellowish brown colour due to the presence of dissolved nitrogen dioxide. Nitric acid slowly decomposes even at room temperature, specially in the presence of sunlight. Brown vapours are seen in the bottle and the nitric acid turns yellowish in colour.

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