Fresh Rust vs Harvest Brown

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Harvest Brown is a little darker, more muted, warmer than Fresh Rust.

Can I use Harvest Brown instead of Fresh Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Harvest Brown is a little darker, more muted, warmer than Fresh Rust.

Can I use Fresh Rust instead of Harvest Brown?

The same difference applies the other way round: Fresh Rust is a little lighter, 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.