ΔE 7.4, close: similar, with a visible difference side by side. USAAF Light Gray is a little darker, more muted, warmer than WWI German Grey-Green Primer.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). USAAF Light Gray is a little darker, more muted, warmer than WWI German Grey-Green Primer.
The same difference applies the other way round: WWI German Grey-Green Primer is a little lighter, more saturated, 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.
See WWI German Grey-Green Primer on the colour wheel See USAAF Light Gray on the colour wheel
All matches: WWI German Grey-Green Primer equivalents · USAAF Light Gray equivalents