Ultramarine Blue vs Neptune's Fury

How close are they?

ΔE 23.5, loose: different enough that you should pick on purpose. Neptune's Fury is noticeably lighter, much more saturated than Ultramarine Blue.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Neptune's Fury is noticeably lighter, much more saturated than Ultramarine Blue.

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

The same difference applies the other way round: Ultramarine Blue is noticeably darker, much more muted.

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.