Which on mixing gives deep brown colour?

Nitric oxide, NO reacts with oxygen to produce nitrogen dioxide. 

$2\mathrm{NO}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{NO}}_2\left(\mathrm{g}\right)$

The reddish-brown colour results from the predominant absorption of light in the blue (400–500 nm) part of the spectrum, however the absorption also occurs at shorter wavelengths in the visible spectrum and in the infrared (at longer wavelengths).

NO and ${\mathrm{O}}_2$ are two colourless gases, combining them produces a deep dark or reddish-brown gas.

Hence, correct option is B.