USMC Green FS 34095 vs Straken Green

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Straken Green is a little lighter, cooler than USMC Green FS 34095.

Can I use Straken Green instead of USMC Green FS 34095?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Straken Green is a little lighter, cooler than USMC Green FS 34095.

Can I use USMC Green FS 34095 instead of Straken Green?

The same difference applies the other way round: USMC Green FS 34095 is a little 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.