Rlyeh Grey vs Pacific Green

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Pacific Green is noticeably lighter, more saturated than Rlyeh Grey.

Can I use Pacific Green instead of Rlyeh Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Pacific Green is noticeably lighter, more saturated than Rlyeh Grey.

Can I use Rlyeh Grey instead of Pacific Green?

The same difference applies the other way round: Rlyeh Grey is noticeably darker, 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.