British Brown vs Dark Mustard Khaki

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Dark Mustard Khaki is a little darker than British Brown.

Can I use Dark Mustard Khaki instead of British Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Dark Mustard Khaki is a little darker than British Brown.

Can I use British Brown instead of Dark Mustard Khaki?

The same difference applies the other way round: British Brown is a little lighter.

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.