Dusty Blue vs Greasy Black

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Greasy Black is noticeably darker, more muted than Dusty Blue.

Can I use Greasy Black instead of Dusty Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Greasy Black is noticeably darker, more muted than Dusty Blue.

Can I use Dusty Blue instead of Greasy Black?

The same difference applies the other way round: Dusty Blue is noticeably lighter, 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.