Cobalt Blue vs Oily Steel

How close are they?

ΔE 19.7, loose: different enough that you should pick on purpose. Oily Steel is noticeably lighter, more muted than Cobalt Blue.

Can I use Oily Steel instead of Cobalt Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Oily Steel is noticeably lighter, more muted than Cobalt Blue.

Can I use Cobalt Blue instead of Oily Steel?

The same difference applies the other way round: Cobalt Blue 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.