Autumn Sage vs Surf Aqua

How close are they?

ΔE 18.8, loose: different enough that you should pick on purpose. Surf Aqua is a little darker, more saturated, cooler than Autumn Sage.

Can I use Surf Aqua instead of Autumn Sage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Surf Aqua is a little darker, more saturated, cooler than Autumn Sage.

Can I use Autumn Sage instead of Surf Aqua?

The same difference applies the other way round: Autumn Sage is a little lighter, more muted, 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.