ΔE 13.6, fair: the same colour family, but not a straight swap. Iridescent Purple/Green is a little lighter, more saturated than Runefang Steel. They are different kinds of paint: Citadel Runefang Steel is a metallic and Tamiya Iridescent Purple/Green is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Iridescent Purple/Green is a little lighter, more saturated than Runefang Steel. They are different kinds of paint.
The same difference applies the other way round: Runefang Steel is a little darker, 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.
See Runefang Steel on the colour wheel See Iridescent Purple/Green on the colour wheel
All matches: Runefang Steel equivalents · Iridescent Purple/Green equivalents