Cygnar Blue Highlight vs Medium Blue

How close are they?

ΔE 24.4, loose: different enough that you should pick on purpose. Medium Blue is noticeably darker, much more muted than Cygnar Blue Highlight.

Can I use Medium Blue instead of Cygnar Blue Highlight?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Medium Blue is noticeably darker, much more muted than Cygnar Blue Highlight.

Can I use Cygnar Blue Highlight instead of Medium Blue?

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