Macragge Blue vs Thunder Blue

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Thunder Blue is noticeably lighter, more saturated than Macragge Blue.

Can I use Thunder Blue instead of Macragge Blue?

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

Can I use Macragge Blue instead of Thunder Blue?

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