Rusty Gold vs Dark Emerald

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Dark Emerald is noticeably darker, cooler than Rusty Gold.

Can I use Dark Emerald instead of Rusty Gold?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Dark Emerald is noticeably darker, cooler than Rusty Gold.

Can I use Rusty Gold instead of Dark Emerald?

The same difference applies the other way round: Rusty Gold is noticeably lighter, 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.