Turquoise vs Dark Reaper

How close are they?

ΔE 8.4, close: similar, with a visible difference side by side. Dark Reaper is more muted, cooler than Turquoise.

Can I use Dark Reaper instead of Turquoise?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dark Reaper is more muted, cooler than Turquoise.

Can I use Turquoise instead of Dark Reaper?

The same difference applies the other way round: Turquoise is 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.