Bone Brown vs Mustard Brown

How close are they?

ΔE 6.2, close: similar, with a visible difference side by side. Mustard Brown is a little darker, more saturated than Bone Brown.

Can I use Mustard Brown instead of Bone Brown?

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

Can I use Bone Brown instead of Mustard Brown?

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