ΔE 10.1, fair: the same colour family, but not a straight swap. Dark Elf Highlight is a little lighter than Regiment Grey.
Can I use Dark Elf Highlight instead of Regiment Grey?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Dark Elf Highlight is a little lighter than Regiment Grey.
Can I use Regiment Grey instead of Dark Elf Highlight?
The same difference applies the other way round: Regiment Grey is a little 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.