Bright Blue vs Lothern Blue

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. Lothern Blue is noticeably darker, warmer than Bright Blue.

Can I use Lothern Blue instead of Bright Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Lothern Blue is noticeably darker, warmer than Bright Blue.

Can I use Bright Blue instead of Lothern Blue?

The same difference applies the other way round: Bright Blue is noticeably lighter, cooler.

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.