RLM 79 (1942) vs Bullwhack Brown

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Bullwhack Brown is more saturated than RLM 79 (1942).

Can I use Bullwhack Brown instead of RLM 79 (1942)?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Bullwhack Brown is more saturated than RLM 79 (1942).

Can I use RLM 79 (1942) instead of Bullwhack Brown?

The same difference applies the other way round: RLM 79 (1942) is more muted.

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.