RLM 81 Version 3 vs Dark Boot Brown

How close are they?

ΔE 7.7, close: similar, with a visible difference side by side. Dark Boot Brown is a little darker, cooler than RLM 81 Version 3.

Can I use Dark Boot Brown instead of RLM 81 Version 3?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dark Boot Brown is a little darker, cooler than RLM 81 Version 3.

Can I use RLM 81 Version 3 instead of Dark Boot Brown?

The same difference applies the other way round: RLM 81 Version 3 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.