A nitro alkane reacts with HONO to give insoluble product in alkali which turns blue on treatment with an alkali. The nitro alkane is 

Secondary nitro alkane forms a blue-colored nitroso-nitro alkane when reacts with nitrous acid. This solution remains blue in NaOH indicating that nitroso-nitro alkane of secondary amine does not reacts with NaOH.

$\underset{\mathrm{Secondary} \mathrm{nitro} \mathrm{alkane}}{{\mathrm{R}}_2\mathrm{CH}-{\mathrm{NO}}_2}+\mathrm{HO}-\mathrm{N}=\mathrm{O}\stackrel{{\mathrm{NaNO}}_2/\mathrm{HCl}}{\to }\underset{\mathrm{Blue} \mathrm{color}}{{\mathrm{R}}_2\mathrm{C}\left(\mathrm{NO}\right)-{\mathrm{NO}}_2}+{\mathrm{H}}_2\mathrm{O}$