ΔE 14.9, fair: the same colour family, but not a straight swap. Underhive Ash is noticeably lighter, more muted, cooler than A-21m Light Yellowish Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Underhive Ash is noticeably lighter, more muted, cooler than A-21m Light Yellowish Brown.
The same difference applies the other way round: A-21m Light Yellowish Brown is noticeably darker, more saturated, 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 A-21m Light Yellowish Brown on the colour wheel See Underhive Ash on the colour wheel
All matches: A-21m Light Yellowish Brown equivalents · Underhive Ash equivalents