ΔE 7.2, close: similar, with a visible difference side by side. Russet Brown is a little darker, cooler than Brown Tone. They are different kinds of paint: Army Painter Brown Tone is a wash/shade and Reaper Russet Brown is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Russet Brown is a little darker, cooler than Brown Tone. They are different kinds of paint.
The same difference applies the other way round: Brown 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 Brown Tone on the colour wheel See Russet Brown on the colour wheel
All matches: Brown Tone equivalents · Russet Brown equivalents