Desert Yellow vs Dark Yellow

How close are they?

ΔE 6.2, close: similar, with a visible difference side by side. Dark Yellow is more muted than Desert Yellow.

Can I use Dark Yellow instead of Desert Yellow?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Dark Yellow is more muted than Desert Yellow.

Can I use Desert Yellow instead of Dark Yellow?

The same difference applies the other way round: Desert Yellow is 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.