ΔE 16.3, loose: different enough that you should pick on purpose. Longbeard Grey is a little lighter, more muted than Hellblau RLM 78.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Longbeard Grey is a little lighter, more muted than Hellblau RLM 78.
The same difference applies the other way round: Hellblau RLM 78 is a little darker, 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 Hellblau RLM 78 on the colour wheel See Longbeard Grey on the colour wheel
All matches: Hellblau RLM 78 equivalents · Longbeard Grey equivalents