ΔE 13.2, fair: the same colour family, but not a straight swap. Light Blue RLM78 is much more muted than Mythic Turquoise.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Light Blue RLM78 is much more muted than Mythic Turquoise.
The same difference applies the other way round: Mythic Turquoise is much 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 Mythic Turquoise on the colour wheel See Light Blue RLM78 on the colour wheel
All matches: Mythic Turquoise equivalents · Light Blue RLM78 equivalents