Turquoise Blue vs Phalanx Blue

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Phalanx Blue is noticeably lighter, more saturated than Turquoise Blue.

Can I use Phalanx Blue instead of Turquoise Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Phalanx Blue is noticeably lighter, more saturated than Turquoise Blue.

Can I use Turquoise Blue instead of Phalanx Blue?

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