Blue vs Mystic Blue

How close are they?

ΔE 25.1, loose: different enough that you should pick on purpose. Mystic Blue is noticeably darker, much more muted than Blue.

Can I use Mystic Blue instead of Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Mystic Blue is noticeably darker, much more muted than Blue.

Can I use Blue instead of Mystic Blue?

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