Macragge Blue vs Blue Base

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Blue Base is noticeably lighter, more muted than Macragge Blue.

Can I use Blue Base instead of Macragge Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Blue Base is noticeably lighter, more muted than Macragge Blue.

Can I use Macragge Blue instead of Blue Base?

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