Phantasmal Blue vs Spectral Blue

How close are they?

ΔE 24.2, loose: different enough that you should pick on purpose. Spectral Blue is noticeably darker, more muted, cooler than Phantasmal Blue.

Can I use Spectral Blue instead of Phantasmal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Spectral Blue is noticeably darker, more muted, cooler than Phantasmal Blue.

Can I use Phantasmal Blue instead of Spectral Blue?

The same difference applies the other way round: Phantasmal Blue is noticeably lighter, more saturated, 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.