Ashen Stone vs Space Grey

How close are they?

ΔE 40.1, loose: different enough that you should pick on purpose. Space Grey is noticeably darker, more saturated than Ashen Stone.

Can I use Space Grey instead of Ashen Stone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 40). Space Grey is noticeably darker, more saturated than Ashen Stone.

Can I use Ashen Stone instead of Space Grey?

The same difference applies the other way round: Ashen Stone 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.