Turnbull Turquoise vs Dark Turquoise

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Dark Turquoise is noticeably lighter, more saturated than Turnbull Turquoise.

Can I use Dark Turquoise instead of Turnbull Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Dark Turquoise is noticeably lighter, more saturated than Turnbull Turquoise.

Can I use Turnbull Turquoise instead of Dark Turquoise?

The same difference applies the other way round: Turnbull Turquoise 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.