Deep Grey vs Coal Black

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Coal Black is a little darker, more saturated, cooler than Deep Grey.

Can I use Coal Black instead of Deep Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Coal Black is a little darker, more saturated, cooler than Deep Grey.

Can I use Deep Grey instead of Coal Black?

The same difference applies the other way round: Deep Grey is a little lighter, more muted, warmer.

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.