Grey-Blue 1920s-1930s vs Light Blue RLM76

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. Light Blue RLM76 is a little darker, cooler than Grey-Blue 1920s-1930s.

Can I use Light Blue RLM76 instead of Grey-Blue 1920s-1930s?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Light Blue RLM76 is a little darker, cooler than Grey-Blue 1920s-1930s.

Can I use Grey-Blue 1920s-1930s instead of Light Blue RLM76?

The same difference applies the other way round: Grey-Blue 1920s-1930s is a little 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.