Grim Rust vs Brilli White

How close are they?

ΔE 76.2, loose: different enough that you should pick on purpose. Brilli White is noticeably lighter, much more muted than Grim Rust.

Can I use Brilli White instead of Grim Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 76). Brilli White is noticeably lighter, much more muted than Grim Rust.

Can I use Grim Rust instead of Brilli White?

The same difference applies the other way round: Grim Rust is noticeably darker, much more saturated.

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.