Autumn Sage vs Longbeard Grey

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Longbeard Grey is a little lighter, much more muted than Autumn Sage.

Can I use Longbeard Grey instead of Autumn Sage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Longbeard Grey is a little lighter, much more muted than Autumn Sage.

Can I use Autumn Sage instead of Longbeard Grey?

The same difference applies the other way round: Autumn Sage is a little darker, much 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.