ΔE 10.5, fair: the same colour family, but not a straight swap. Underhive Ash is a little darker, warmer than WWI German Grey-Green Primer.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Underhive Ash is a little darker, warmer than WWI German Grey-Green Primer.
The same difference applies the other way round: WWI German Grey-Green Primer is a little lighter, 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 WWI German Grey-Green Primer on the colour wheel See Underhive Ash on the colour wheel
All matches: WWI German Grey-Green Primer equivalents · Underhive Ash equivalents