RLM 78 Light Blue vs Russian AF Blue

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Russian AF Blue is a little darker, more saturated than RLM 78 Light Blue.

Can I use Russian AF Blue instead of RLM 78 Light Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Russian AF Blue is a little darker, more saturated than RLM 78 Light Blue.

Can I use RLM 78 Light Blue instead of Russian AF Blue?

The same difference applies the other way round: RLM 78 Light Blue is a little lighter, 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.