Dark Emerald vs Gun Metal

How close are they?

ΔE 30.9, loose: different enough that you should pick on purpose. Gun Metal is noticeably lighter, much more muted than Dark Emerald.

Can I use Gun Metal instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Gun Metal is noticeably lighter, much more muted than Dark Emerald.

Can I use Dark Emerald instead of Gun Metal?

The same difference applies the other way round: Dark Emerald is noticeably darker, much more saturated.

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.