Ocean Blue vs Dark Reaper

How close are they?

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

Can I use Dark Reaper instead of Ocean Blue?

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

Can I use Ocean Blue instead of Dark Reaper?

The same difference applies the other way round: Ocean Blue is 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.