ΔE 10.2, fair: the same colour family, but not a straight swap. Obsidian Black is a little lighter than Grey Knights Steel.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Obsidian Black is a little lighter than Grey Knights Steel.
The same difference applies the other way round: Grey Knights Steel 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.
See Grey Knights Steel on the colour wheel See Obsidian Black on the colour wheel
All matches: Grey Knights Steel equivalents · Obsidian Black equivalents