Lothern Blue vs Pale Indigo

How close are they?

ΔE 17.2, loose: different enough that you should pick on purpose. Pale Indigo is a little darker, warmer than Lothern Blue.

Can I use Pale Indigo instead of Lothern Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Pale Indigo is a little darker, warmer than Lothern Blue.

Can I use Lothern Blue instead of Pale Indigo?

The same difference applies the other way round: Lothern Blue is a little 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.