ΔE 10.9, fair: the same colour family, but not a straight swap. Medium Grey Blue is a little lighter, more saturated, cooler than WWI German Fokker Grey.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Medium Grey Blue is a little lighter, more saturated, cooler than WWI German Fokker Grey.
The same difference applies the other way round: WWI German Fokker Grey is a little darker, more muted, warmer.
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 WWI German Fokker Grey on the colour wheel See Medium Grey Blue on the colour wheel
All matches: WWI German Fokker Grey equivalents · Medium Grey Blue equivalents