Hexwraith Flame vs Cool Blue

How close are they?

ΔE 25.1, loose: different enough that you should pick on purpose. Cool Blue is a little darker, much more muted, cooler than Hexwraith Flame.

Can I use Cool Blue instead of Hexwraith Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Cool Blue is a little darker, much more muted, cooler than Hexwraith Flame.

Can I use Hexwraith Flame instead of Cool Blue?

The same difference applies the other way round: Hexwraith Flame is a little 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.