Brilli White vs Green Rust

How close are they?

ΔE 37.1, loose: different enough that you should pick on purpose. Green Rust is noticeably darker, much more saturated than Brilli White.

Can I use Green Rust instead of Brilli White?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Green Rust is noticeably darker, much more saturated than Brilli White.

Can I use Brilli White instead of Green Rust?

The same difference applies the other way round: Brilli White is noticeably lighter, much more muted.

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.