Sapphire Blue vs Unders. Blue “Faded”

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Unders. Blue “Faded” is noticeably darker, cooler than Sapphire Blue.

Can I use Unders. Blue “Faded” instead of Sapphire Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Unders. Blue “Faded” is noticeably darker, cooler than Sapphire Blue.

Can I use Sapphire Blue instead of Unders. Blue “Faded”?

The same difference applies the other way round: Sapphire Blue is noticeably lighter, 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.