Fenrisian Grey vs Sky Grey

How close are they?

ΔE 19.5, loose: different enough that you should pick on purpose. Sky Grey is noticeably lighter, more muted than Fenrisian Grey.

Can I use Sky Grey instead of Fenrisian Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Sky Grey is noticeably lighter, more muted than Fenrisian Grey.

Can I use Fenrisian Grey instead of Sky Grey?

The same difference applies the other way round: Fenrisian Grey is noticeably darker, more saturated.

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.