Deep Sky Blue vs Turquoise

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Turquoise is noticeably darker, more saturated, cooler than Deep Sky Blue.

Can I use Turquoise instead of Deep Sky Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Turquoise is noticeably darker, more saturated, cooler than Deep Sky Blue.

Can I use Deep Sky Blue instead of Turquoise?

The same difference applies the other way round: Deep Sky Blue is noticeably lighter, more muted, warmer.

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.