Celestium Blue vs Storm Blue

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Storm Blue is noticeably darker, warmer than Celestium Blue.

Can I use Storm Blue instead of Celestium Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Storm Blue is noticeably darker, warmer than Celestium Blue.

Can I use Celestium Blue instead of Storm Blue?

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