Shieldwall Blue vs Dark Turquoise

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Dark Turquoise is noticeably darker, more muted, warmer than Shieldwall Blue.

Can I use Dark Turquoise instead of Shieldwall Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Dark Turquoise is noticeably darker, more muted, warmer than Shieldwall Blue.

Can I use Shieldwall Blue instead of Dark Turquoise?

The same difference applies the other way round: Shieldwall Blue is noticeably lighter, more saturated, cooler.

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.