Phantasmal Blue vs Mythic Turquoise

How close are they?

ΔE 18.1, loose: different enough that you should pick on purpose. Mythic Turquoise is a little darker, much more muted, warmer than Phantasmal Blue.

Can I use Mythic Turquoise instead of Phantasmal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Mythic Turquoise is a little darker, much more muted, warmer than Phantasmal Blue.

Can I use Phantasmal Blue instead of Mythic Turquoise?

The same difference applies the other way round: Phantasmal Blue is a little lighter, much more saturated, cooler.

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.