Etherium Blue vs Sky Blue

How close are they?

ΔE 19.6, loose: different enough that you should pick on purpose. Sky Blue is noticeably darker, much more saturated than Etherium Blue.

Can I use Sky Blue instead of Etherium Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Sky Blue is noticeably darker, much more saturated than Etherium Blue.

Can I use Etherium Blue instead of Sky Blue?

The same difference applies the other way round: Etherium Blue is noticeably lighter, much 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.