Ice Storm vs Sky Blue

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Sky Blue is a little lighter, warmer than Ice Storm.

Can I use Sky Blue instead of Ice Storm?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Sky Blue is a little lighter, warmer than Ice Storm.

Can I use Ice Storm instead of Sky Blue?

The same difference applies the other way round: Ice Storm is a little darker, 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.