Nymph Green vs Gauss Green

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Gauss Green is noticeably lighter, more saturated, warmer than Nymph Green.

Can I use Gauss Green instead of Nymph Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Gauss Green is noticeably lighter, more saturated, warmer than Nymph Green.

Can I use Nymph Green instead of Gauss Green?

The same difference applies the other way round: Nymph Green is noticeably darker, more muted, 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.