Grey-Blue 1920s-1930s vs Underhive Ash

How close are they?

ΔE 10.6, fair: the same colour family, but not a straight swap. Underhive Ash is warmer than Grey-Blue 1920s-1930s.

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

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Underhive Ash is warmer than Grey-Blue 1920s-1930s.

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

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

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.