Turquoise Deep vs Light Turquoise

How close are they?

ΔE 22.6, loose: different enough that you should pick on purpose. Light Turquoise is noticeably lighter, more saturated, cooler than Turquoise Deep.

Can I use Light Turquoise instead of Turquoise Deep?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Light Turquoise is noticeably lighter, more saturated, cooler than Turquoise Deep.

Can I use Turquoise Deep instead of Light Turquoise?

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