ΔE 7.9, close: similar, with a visible difference side by side. IJN Deep Dark Green is a little darker, more saturated than Rlyeh Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). IJN Deep Dark Green is a little darker, more saturated than Rlyeh Grey.
The same difference applies the other way round: Rlyeh Grey is a little lighter, more muted.
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.
See Rlyeh Grey on the colour wheel See IJN Deep Dark Green on the colour wheel
All matches: Rlyeh Grey equivalents · IJN Deep Dark Green equivalents