ΔE 15.7, loose: different enough that you should pick on purpose. RLM 78 Light Blue is noticeably darker, cooler than Neptune Glow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). RLM 78 Light Blue is noticeably darker, cooler than Neptune Glow.
The same difference applies the other way round: Neptune Glow is noticeably lighter, 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 Neptune Glow on the colour wheel See RLM 78 Light Blue on the colour wheel
All matches: Neptune Glow equivalents · RLM 78 Light Blue equivalents