Black Rose Immortal vs Flat Brown

How close are they?

ΔE 7.9, close: similar, with a visible difference side by side. Flat Brown is a little darker, cooler than Black Rose Immortal.

Can I use Flat Brown instead of Black Rose Immortal?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Brown is a little darker, cooler than Black Rose Immortal.

Can I use Black Rose Immortal instead of Flat Brown?

The same difference applies the other way round: Black Rose Immortal 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.