Light Feldgrau vs Bright Green Blue

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Bright Green Blue is noticeably lighter, cooler than Light Feldgrau.

Can I use Bright Green Blue instead of Light Feldgrau?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Bright Green Blue is noticeably lighter, cooler than Light Feldgrau.

Can I use Light Feldgrau instead of Bright Green Blue?

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