ΔE 12.0, fair: the same colour family, but not a straight swap. Brown Gray is a little darker than Typhon Ash.
Can I use Brown Gray instead of Typhon Ash?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Brown Gray is a little darker than Typhon Ash.
Can I use Typhon Ash instead of Brown Gray?
The same difference applies the other way round: Typhon Ash is a little lighter.
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.