Cobalt Blue vs Rough Iron

How close are they?

ΔE 22.2, loose: different enough that you should pick on purpose. Rough Iron is noticeably lighter, much more muted than Cobalt Blue.

Can I use Rough Iron instead of Cobalt Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Rough Iron is noticeably lighter, much more muted than Cobalt Blue.

Can I use Cobalt Blue instead of Rough Iron?

The same difference applies the other way round: Cobalt Blue is noticeably darker, much 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.