Dark Yellow vs Dirty Grey

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Dirty Grey is noticeably darker, more muted, cooler than Dark Yellow.

Can I use Dirty Grey instead of Dark Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dirty Grey is noticeably darker, more muted, cooler than Dark Yellow.

Can I use Dark Yellow instead of Dirty Grey?

The same difference applies the other way round: Dark Yellow is noticeably lighter, more saturated, 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.