ΔE 11.6, fair: the same colour family, but not a straight swap. Green Ink is noticeably darker, warmer than Guardian Green. They are different kinds of paint: Army Painter Guardian Green is an opaque base/layer paint and Reaper Green Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Green Ink is noticeably darker, warmer than Guardian Green. They are different kinds of paint.
The same difference applies the other way round: Guardian Green is noticeably lighter, cooler.
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 Guardian Green on the colour wheel See Green Ink on the colour wheel
All matches: Guardian Green equivalents · Green Ink equivalents