ΔE 7.6, close: similar, with a visible difference side by side. Grey Castle is a little darker, more muted than Ruins Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Grey Castle is a little darker, more muted than Ruins Grey.
The same difference applies the other way round: Ruins Grey is a little lighter, 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 Ruins Grey on the colour wheel See Grey Castle on the colour wheel
All matches: Ruins Grey equivalents · Grey Castle equivalents