Lothern Blue vs Sky Blue

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Sky Blue is noticeably lighter, more saturated than Lothern Blue.

Can I use Sky Blue instead of Lothern Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Sky Blue is noticeably lighter, more saturated than Lothern Blue.

Can I use Lothern Blue instead of Sky Blue?

The same difference applies the other way round: Lothern 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.