Calgar Blue vs Relic Blue

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Relic Blue is noticeably lighter, more muted, cooler than Calgar Blue.

Can I use Relic Blue instead of Calgar Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Relic Blue is noticeably lighter, more muted, cooler than Calgar Blue.

Can I use Calgar Blue instead of Relic Blue?

The same difference applies the other way round: Calgar Blue is noticeably darker, more saturated, 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.