Pure Filth vs Ushabti Bone

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Ushabti Bone is noticeably lighter, cooler than Pure Filth.

Can I use Ushabti Bone instead of Pure Filth?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Ushabti Bone is noticeably lighter, cooler than Pure Filth.

Can I use Pure Filth instead of Ushabti Bone?

The same difference applies the other way round: Pure Filth is noticeably darker, 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.