Ryza Rust vs Yellow RLM04

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Yellow RLM04 is a little lighter than Ryza Rust.

Can I use Yellow RLM04 instead of Ryza Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Yellow RLM04 is a little lighter than Ryza Rust.

Can I use Ryza Rust instead of Yellow RLM04?

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

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.