ΔE 18.6, loose: different enough that you should pick on purpose. Summoning Sickness is a little lighter, much more saturated, cooler than Runelord Brass.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Summoning Sickness is a little lighter, much more saturated, cooler than Runelord Brass.
The same difference applies the other way round: Runelord Brass is a little darker, much more muted, warmer.
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 Summoning Sickness on the colour wheel
All matches: Runelord Brass equivalents · Summoning Sickness equivalents