ΔE 6.6, close: similar, with a visible difference side by side. Army Green is a little darker, cooler than Rock Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Army Green is a little darker, cooler than Rock Grey.
The same difference applies the other way round: Rock Grey is a little lighter, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Rock Grey on the colour wheel See Army Green on the colour wheel
All matches: Rock Grey equivalents · Army Green equivalents