Ghoul Green vs Cool Blue

How close are they?

ΔE 35.6, loose: different enough that you should pick on purpose. Cool Blue is noticeably darker, much more muted, cooler than Ghoul Green.

Can I use Cool Blue instead of Ghoul Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Cool Blue is noticeably darker, much more muted, cooler than Ghoul Green.

Can I use Ghoul Green instead of Cool Blue?

The same difference applies the other way round: Ghoul Green is noticeably lighter, much more saturated, warmer.

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.