ΔE 11.9, fair: the same colour family, but not a straight swap. Medium Sea Gray (RAF) is a little lighter than Grey Castle.
Can I use Medium Sea Gray (RAF) instead of Grey Castle?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Medium Sea Gray (RAF) is a little lighter than Grey Castle.
Can I use Grey Castle instead of Medium Sea Gray (RAF)?
The same difference applies the other way round: Grey Castle is a little darker.
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.