Light Feldgrau vs Donkey Grey

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Donkey Grey is noticeably lighter, more muted than Light Feldgrau.

Can I use Donkey Grey instead of Light Feldgrau?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Donkey Grey is noticeably lighter, more muted than Light Feldgrau.

Can I use Light Feldgrau instead of Donkey Grey?

The same difference applies the other way round: Light Feldgrau is noticeably darker, 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.