Deep Sky Blue vs Fenrisian Grey

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Fenrisian Grey is a little darker, more muted than Deep Sky Blue.

Can I use Fenrisian Grey instead of Deep Sky Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Fenrisian Grey is a little darker, more muted than Deep Sky Blue.

Can I use Deep Sky Blue instead of Fenrisian Grey?

The same difference applies the other way round: Deep Sky Blue is a little lighter, 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.