Electric Blue vs Primary Cyan

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Primary Cyan is noticeably darker, warmer than Electric Blue.

Can I use Primary Cyan instead of Electric Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Primary Cyan is noticeably darker, warmer than Electric Blue.

Can I use Electric Blue instead of Primary Cyan?

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