ΔE 8.2, close: similar, with a visible difference side by side. Steel Blue is a little darker, more saturated, warmer than Extra Dark Seagray BS381C/640.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Steel Blue is a little darker, more saturated, warmer than Extra Dark Seagray BS381C/640.
The same difference applies the other way round: Extra Dark Seagray BS381C/640 is a little lighter, more muted, 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 Extra Dark Seagray BS381C/640 on the colour wheel See Steel Blue on the colour wheel
All matches: Extra Dark Seagray BS381C/640 equivalents · Steel Blue equivalents