ΔE 20.5, loose: different enough that you should pick on purpose. N8 Neutral Gray is noticeably lighter, more muted than Grey Castle.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). N8 Neutral Gray is noticeably lighter, more muted than Grey Castle.
The same difference applies the other way round: Grey Castle is noticeably darker, 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 Grey Castle on the colour wheel See N8 Neutral Gray on the colour wheel
All matches: Grey Castle equivalents · N8 Neutral Gray equivalents