ΔE 10.5, fair: the same colour family, but not a straight swap. Medium Grey Blue is more saturated than RLM 78 (1942).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Medium Grey Blue is more saturated than RLM 78 (1942).
The same difference applies the other way round: RLM 78 (1942) is 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 RLM 78 (1942) on the colour wheel See Medium Grey Blue on the colour wheel
All matches: RLM 78 (1942) equivalents · Medium Grey Blue equivalents