Bright Slate Grey vs Space Grey

How close are they?

ΔE 28.9, loose: different enough that you should pick on purpose. Space Grey is noticeably darker, more saturated than Bright Slate Grey.

Can I use Space Grey instead of Bright Slate Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Space Grey is noticeably darker, more saturated than Bright Slate Grey.

Can I use Bright Slate Grey instead of Space Grey?

The same difference applies the other way round: Bright Slate Grey is noticeably 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.