Steel vs Grave Robber

How close are they?

ΔE 18.1, loose: different enough that you should pick on purpose. Grave Robber is a little darker, much more saturated than Steel.

Can I use Grave Robber instead of Steel?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Grave Robber is a little darker, much more saturated than Steel.

Can I use Steel instead of Grave Robber?

The same difference applies the other way round: Steel is a little lighter, much 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.