Phthalo Turquoise Light vs Steel Grey

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Steel Grey is noticeably lighter, more muted than Phthalo Turquoise Light.

Can I use Steel Grey instead of Phthalo Turquoise Light?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Steel Grey is noticeably lighter, more muted than Phthalo Turquoise Light.

Can I use Phthalo Turquoise Light instead of Steel Grey?

The same difference applies the other way round: Phthalo Turquoise Light is noticeably darker, 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.