ΔE 15.4, loose: different enough that you should pick on purpose. IJA Light Grey Green is more muted, cooler than Thar Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). IJA Light Grey Green is more muted, cooler than Thar Brown.
The same difference applies the other way round: Thar Brown is more saturated, 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 Thar Brown on the colour wheel See IJA Light Grey Green on the colour wheel
All matches: Thar Brown equivalents · IJA Light Grey Green equivalents