Grim Rust vs Orange Neon

How close are they?

ΔE 34.3, loose: different enough that you should pick on purpose. Orange Neon is a little lighter, much more saturated, cooler than Grim Rust.

Can I use Orange Neon instead of Grim Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Orange Neon is a little lighter, much more saturated, cooler than Grim Rust.

Can I use Grim Rust instead of Orange Neon?

The same difference applies the other way round: Grim Rust is a little darker, 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.