Cygnar Blue Highlight vs Imperial Blue

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Imperial Blue is noticeably darker, more muted than Cygnar Blue Highlight.

Can I use Imperial Blue instead of Cygnar Blue Highlight?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Imperial Blue is noticeably darker, more muted than Cygnar Blue Highlight.

Can I use Cygnar Blue Highlight instead of Imperial Blue?

The same difference applies the other way round: Cygnar Blue Highlight is noticeably lighter, more saturated.

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.