Light Rust vs Flat Yellow

How close are they?

ΔE 8.1, close: similar, with a visible difference side by side. Flat Yellow is a little darker, more muted than Light Rust.

Can I use Flat Yellow instead of Light Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Yellow is a little darker, more muted than Light Rust.

Can I use Light Rust instead of Flat Yellow?

The same difference applies the other way round: Light Rust is a little lighter, 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.