ΔE 16.7, loose: different enough that you should pick on purpose. Ultramarine Blue is much more muted than Cerulean Blue, Chromium.
Can I use Ultramarine Blue instead of Cerulean Blue, Chromium?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Ultramarine Blue is much more muted than Cerulean Blue, Chromium.
Can I use Cerulean Blue, Chromium instead of Ultramarine Blue?
The same difference applies the other way round: Cerulean Blue, Chromium is much more saturated.
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.