Grey-Blue 1920s-1930s vs Longbeard Grey

How close are they?

ΔE 8.7, close: similar, with a visible difference side by side. Longbeard Grey is more muted than Grey-Blue 1920s-1930s.

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

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Longbeard Grey is more muted than Grey-Blue 1920s-1930s.

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

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