Moldy Ochre vs German Yellow

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. German Yellow is much more muted, cooler than Moldy Ochre.

Can I use German Yellow instead of Moldy Ochre?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). German Yellow is much more muted, cooler than Moldy Ochre.

Can I use Moldy Ochre instead of German Yellow?

The same difference applies the other way round: Moldy Ochre is much 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.