Basilicanum Grey vs Space Grey

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Space Grey is noticeably darker, much more saturated than Basilicanum Grey.

Can I use Space Grey instead of Basilicanum Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Space Grey is noticeably darker, much more saturated than Basilicanum Grey.

Can I use Basilicanum Grey instead of Space 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.