ΔE 6.8, close: similar, with a visible difference side by side. Grey Castle is more muted than Medium Sea Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Grey Castle is more muted than Medium Sea Grey.
The same difference applies the other way round: Medium Sea Grey is more saturated.
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 Medium Sea Grey on the colour wheel See Grey Castle on the colour wheel
All matches: Medium Sea Grey equivalents · Grey Castle equivalents