Gobi Brown vs Dirty Grey

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Dirty Grey is noticeably lighter, more saturated than Gobi Brown.

Can I use Dirty Grey instead of Gobi Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Dirty Grey is noticeably lighter, more saturated than Gobi Brown.

Can I use Gobi Brown instead of Dirty Grey?

The same difference applies the other way round: Gobi Brown is noticeably darker, more muted.

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.