AMT-11 Blue-Grey vs Dark Reaper

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Dark Reaper is a little darker, warmer than AMT-11 Blue-Grey.

Can I use Dark Reaper instead of AMT-11 Blue-Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Dark Reaper is a little darker, warmer than AMT-11 Blue-Grey.

Can I use AMT-11 Blue-Grey instead of Dark Reaper?

The same difference applies the other way round: AMT-11 Blue-Grey is a little lighter, cooler.

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.