ΔE 12.5, fair: the same colour family, but not a straight swap. Steel Legion Drab is a little lighter, warmer than Field Grey Base #1 (Green uniform).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Steel Legion Drab is a little lighter, warmer than Field Grey Base #1 (Green uniform).
The same difference applies the other way round: Field Grey Base #1 (Green uniform) is a little darker, 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 Field Grey Base #1 (Green uniform) on the colour wheel See Steel Legion Drab on the colour wheel
All matches: Field Grey Base #1 (Green uniform) equivalents · Steel Legion Drab equivalents