Dark Mustard Khaki vs Ground Brown

How close are they?

ΔE 20.3, loose: different enough that you should pick on purpose. Ground Brown is much more muted, warmer than Dark Mustard Khaki.

Can I use Ground Brown instead of Dark Mustard Khaki?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ground Brown is much more muted, warmer than Dark Mustard Khaki.

Can I use Dark Mustard Khaki instead of Ground Brown?

The same difference applies the other way round: Dark Mustard Khaki is much more saturated, cooler.

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.