Concentrated nitric acid is colorless when pure. However, it often appears yellowish or even brownish due to the presence of dissolved nitrogen dioxide ($NO_2$).

Here's why:

* Nitric acid decomposes slightly: Even in the absence of light, nitric acid can decompose into nitrogen dioxide, water, and oxygen:

```

4 HNO₃ → 4 NO₂ + 2 H₂O + O₂

```

* Nitrogen dioxide dissolves in the acid: The resulting nitrogen dioxide dissolves in the nitric acid, giving it a yellow or brown color.

* Exposure to light accelerates decomposition: Exposure to light accelerates the decomposition process, making the discoloration more pronounced.

So, while pure nitric acid is colorless, the yellow or brown color you often see is a sign of decomposition and the presence of dissolved nitrogen dioxide.