ΔE 30.7, loose: different enough that you should pick on purpose. Ultramarine Blue is much more saturated than Sir Coates Silver.
Can I use Ultramarine Blue instead of Sir Coates Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Ultramarine Blue is much more saturated than Sir Coates Silver.
Can I use Sir Coates Silver instead of Ultramarine Blue?
The same difference applies the other way round: Sir Coates Silver is 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.