ΔE 10.1, fair: the same colour family, but not a straight swap. Red Ink is a little darker, cooler than Rust Tone. They are different kinds of paint: Army Painter Rust Tone is a wash/shade and Reaper Red Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Red Ink is a little darker, cooler than Rust Tone. They are different kinds of paint.
The same difference applies the other way round: Rust Tone is a little lighter, 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.
See Rust Tone on the colour wheel See Red Ink on the colour wheel
All matches: Rust Tone equivalents · Red Ink equivalents