Sky Blue vs Aegis Aqua

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Aegis Aqua is a little darker, more saturated, cooler than Sky Blue.

Can I use Aegis Aqua instead of Sky Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Aegis Aqua is a little darker, more saturated, cooler than Sky Blue.

Can I use Sky Blue instead of Aegis Aqua?

The same difference applies the other way round: Sky Blue is a little lighter, more muted, warmer.

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.