ΔE 12.1, fair: the same colour family, but not a straight swap. Muddy Olive is noticeably lighter than Green Tone. They are different kinds of paint: Army Painter Green Tone is a wash/shade and Reaper Muddy Olive is an opaque base/layer 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). Muddy Olive is noticeably lighter than Green Tone. They are different kinds of paint.
The same difference applies the other way round: Green Tone is noticeably darker.
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 Green Tone on the colour wheel See Muddy Olive on the colour wheel
All matches: Green Tone equivalents · Muddy Olive equivalents