Regal Blue vs Mediterranean Blue

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Mediterranean Blue is noticeably lighter, more saturated than Regal Blue.

Can I use Mediterranean Blue instead of Regal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mediterranean Blue is noticeably lighter, more saturated than Regal Blue.

Can I use Regal Blue instead of Mediterranean Blue?

The same difference applies the other way round: Regal 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.