Neptune's Fury vs Calgar Blue

How close are they?

ΔE 22.6, loose: different enough that you should pick on purpose. Calgar Blue is noticeably darker, much more muted, cooler than Neptune's Fury.

Can I use Calgar Blue instead of Neptune's Fury?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Calgar Blue is noticeably darker, much more muted, cooler than Neptune's Fury.

Can I use Neptune's Fury instead of Calgar Blue?

The same difference applies the other way round: Neptune's Fury is noticeably lighter, much 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.