Turnbull Turquoise vs Deep Black

How close are they?

ΔE 18.7, loose: different enough that you should pick on purpose. Deep Black is noticeably darker, more muted than Turnbull Turquoise.

Can I use Deep Black instead of Turnbull Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Deep Black is noticeably darker, more muted than Turnbull Turquoise.

Can I use Turnbull Turquoise instead of Deep Black?

The same difference applies the other way round: Turnbull Turquoise is noticeably lighter, 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.