ΔE 14.9, fair: the same colour family, but not a straight swap. Obsidian Black is noticeably lighter than Light Gun Metal.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Obsidian Black is noticeably lighter than Light Gun Metal.
The same difference applies the other way round: Light Gun Metal 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 Light Gun Metal on the colour wheel See Obsidian Black on the colour wheel
All matches: Light Gun Metal equivalents · Obsidian Black equivalents