Sapphire Gem vs Neptune's Fury

How close are they?

ΔE 18.4, loose: different enough that you should pick on purpose. Neptune's Fury is a little lighter, much more saturated, warmer than Sapphire Gem.

Can I use Neptune's Fury instead of Sapphire Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Neptune's Fury is a little lighter, much more saturated, warmer than Sapphire Gem.

Can I use Sapphire Gem instead of Neptune's Fury?

The same difference applies the other way round: Sapphire Gem is a little darker, much more muted, 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.