Red Brown vs Tree Ancient

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Tree Ancient is a little darker, more muted than Red Brown.

Can I use Tree Ancient instead of Red Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Tree Ancient is a little darker, more muted than Red Brown.

Can I use Red Brown instead of Tree Ancient?

The same difference applies the other way round: Red Brown 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.