Desert Yellow vs Blackert Brown

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Blackert Brown is noticeably lighter, warmer than Desert Yellow.

Can I use Blackert Brown instead of Desert Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Blackert Brown is noticeably lighter, warmer than Desert Yellow.

Can I use Desert Yellow instead of Blackert Brown?

The same difference applies the other way round: Desert Yellow is noticeably darker, cooler.

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.