Autumn Sage vs Tropical Turquoise

How close are they?

ΔE 21.1, loose: different enough that you should pick on purpose. Tropical Turquoise is much more saturated, cooler than Autumn Sage.

Can I use Tropical Turquoise instead of Autumn Sage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Tropical Turquoise is much more saturated, cooler than Autumn Sage.

Can I use Autumn Sage instead of Tropical Turquoise?

The same difference applies the other way round: Autumn Sage is much 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.