Grey-Blue vs Deep Sky Blue

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Deep Sky Blue is noticeably darker, more saturated than Grey-Blue.

Can I use Deep Sky Blue instead of Grey-Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Deep Sky Blue is noticeably darker, more saturated than Grey-Blue.

Can I use Grey-Blue instead of Deep Sky Blue?

The same difference applies the other way round: Grey-Blue is noticeably lighter, 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.