Autumn Sage vs Underhive Ash

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Underhive Ash is more muted, warmer than Autumn Sage.

Can I use Underhive Ash instead of Autumn Sage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Underhive Ash is more muted, warmer than Autumn Sage.

Can I use Autumn Sage instead of Underhive Ash?

The same difference applies the other way round: Autumn Sage is more saturated, 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.