ΔE 22.9, loose: different enough that you should pick on purpose. Monastic Green is a little darker, much more saturated than Graphite Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Monastic Green is a little darker, much more saturated than Graphite Grey.
The same difference applies the other way round: Graphite Grey is a little lighter, much more muted.
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 Graphite Grey on the colour wheel See Monastic Green on the colour wheel
All matches: Graphite Grey equivalents · Monastic Green equivalents