RLM 76 Version 1 vs USAF Light Gray

How close are they?

ΔE 6.2, close: similar, with a visible difference side by side. USAF Light Gray is a little lighter, more muted than RLM 76 Version 1.

Can I use USAF Light Gray instead of RLM 76 Version 1?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). USAF Light Gray is a little lighter, more muted than RLM 76 Version 1.

Can I use RLM 76 Version 1 instead of USAF Light Gray?

The same difference applies the other way round: RLM 76 Version 1 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.