ΔE 20.6, loose: different enough that you should pick on purpose. Iridescent Gold Deep (Fine) is noticeably lighter, more muted, warmer than Runelord Brass. They are different kinds of paint: Citadel Runelord Brass is a metallic and Golden Iridescent Gold Deep (Fine) is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Iridescent Gold Deep (Fine) is noticeably lighter, more muted, warmer than Runelord Brass. They are different kinds of paint.
The same difference applies the other way round: Runelord Brass is noticeably darker, more saturated, cooler.
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 Runelord Brass on the colour wheel See Iridescent Gold Deep (Fine) on the colour wheel
All matches: Runelord Brass equivalents · Iridescent Gold Deep (Fine) equivalents