Stratos Blue vs Turquois (Phthalo)

How close are they?

ΔE 22.5, loose: different enough that you should pick on purpose. Turquois (Phthalo) is a little darker, more muted than Stratos Blue.

Can I use Turquois (Phthalo) instead of Stratos Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Turquois (Phthalo) is a little darker, more muted than Stratos Blue.

Can I use Stratos Blue instead of Turquois (Phthalo)?

The same difference applies the other way round: Stratos Blue is a little 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.