Macragge Blue vs Blue

How close are they?

ΔE 17.5, loose: different enough that you should pick on purpose. Blue is noticeably lighter, more saturated, cooler than Macragge Blue.

Can I use Blue instead of Macragge Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Blue is noticeably lighter, more saturated, cooler than Macragge Blue.

Can I use Macragge Blue instead of Blue?

The same difference applies the other way round: Macragge Blue is noticeably darker, more muted, warmer.

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.