Which oxide of nitrogen is a coloured gas?

Why is nitrogen chemically inert?

Hydrolysis of $PCl_5$ gives

Among the oxides of nitrogen,$N_2O$,$NO$,and $NO_2$,molecules with unpaired electrons are:

What are $X$ and $Y$,respectively in the following reactions?
$Au + \text{aqua regia} \longrightarrow AuCl_4^{-} + H_2O + X$
$Pt + \text{aqua regia} \longrightarrow PtCl_6^{2-} + H_2O + Y$

Chemical nature of the nitrogen oxide compound obtained from a reaction of concentrated nitric acid and $P_{4}O_{10}$ (in $4:1$ ratio) is: