Cosmic Dust vs Ushabti Bone

How close are they?

ΔE 26.4, loose: different enough that you should pick on purpose. Ushabti Bone is noticeably darker, much more muted than Cosmic Dust.

Can I use Ushabti Bone instead of Cosmic Dust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Ushabti Bone is noticeably darker, much more muted than Cosmic Dust.

Can I use Cosmic Dust instead of Ushabti Bone?

The same difference applies the other way round: Cosmic Dust is noticeably lighter, much more saturated.

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.