ΔE 16.6, loose: different enough that you should pick on purpose. Mythril Blade is noticeably lighter than Grey Knights Steel.
Can I use Mythril Blade instead of Grey Knights Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Mythril Blade is noticeably lighter than Grey Knights Steel.
Can I use Grey Knights Steel instead of Mythril Blade?
The same difference applies the other way round: Grey Knights Steel is noticeably darker.
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.