Coal Black vs Blue Green

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Blue Green is noticeably lighter, more saturated than Coal Black.

Can I use Blue Green instead of Coal Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Blue Green is noticeably lighter, more saturated than Coal Black.

Can I use Coal Black instead of Blue Green?

The same difference applies the other way round: Coal Black is noticeably darker, 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.