ΔE 23.7, loose: different enough that you should pick on purpose. Heretic Turquoise is noticeably darker, much more saturated than Basilicanum Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Heretic Turquoise is noticeably darker, much more saturated than Basilicanum Grey.
The same difference applies the other way round: Basilicanum Grey is noticeably lighter, much 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 Basilicanum Grey on the colour wheel See Heretic Turquoise on the colour wheel
All matches: Basilicanum Grey equivalents · Heretic Turquoise equivalents