Rough Iron vs Celestial Violet

How close are they?

ΔE 44.0, loose: different enough that you should pick on purpose. Celestial Violet is a little darker, much more saturated than Rough Iron.

Can I use Celestial Violet instead of Rough Iron?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 44). Celestial Violet is a little darker, much more saturated than Rough Iron.

Can I use Rough Iron instead of Celestial Violet?

The same difference applies the other way round: Rough Iron 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.