Cobalt Blue vs Ultramarine Blue

How close are they?

ΔE 25.6, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably darker, warmer than Cobalt Blue.

Can I use Ultramarine Blue instead of Cobalt Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Ultramarine Blue is noticeably darker, warmer than Cobalt Blue.

Can I use Cobalt Blue instead of Ultramarine Blue?

The same difference applies the other way round: Cobalt Blue is noticeably lighter, 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.