ΔE 7.1, close: similar, with a visible difference side by side. Grey Castle is a little darker, more saturated than RAF Sky Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Grey Castle is a little darker, more saturated than RAF Sky Grey.
The same difference applies the other way round: RAF Sky Grey is a little lighter, more muted.
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 RAF Sky Grey on the colour wheel See Grey Castle on the colour wheel
All matches: RAF Sky Grey equivalents · Grey Castle equivalents