ΔE 23.0, loose: different enough that you should pick on purpose. Light Ultramarine Blue is noticeably lighter, more saturated than Sapphire Gem.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Light Ultramarine Blue is noticeably lighter, more saturated than Sapphire Gem.
The same difference applies the other way round: Sapphire Gem is noticeably darker, 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.
See Sapphire Gem on the colour wheel See Light Ultramarine Blue on the colour wheel
All matches: Sapphire Gem equivalents · Light Ultramarine Blue equivalents