Fenrisian Grey vs Glazier Blue

How close are they?

ΔE 17.0, loose: different enough that you should pick on purpose. Glazier Blue is noticeably lighter, warmer than Fenrisian Grey.

Can I use Glazier Blue instead of Fenrisian Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Glazier Blue is noticeably lighter, warmer than Fenrisian Grey.

Can I use Fenrisian Grey instead of Glazier Blue?

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

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.