Goblin Green vs Bright Yellow Green

How close are they?

ΔE 27.6, loose: different enough that you should pick on purpose. Bright Yellow Green is noticeably lighter, warmer than Goblin Green.

Can I use Bright Yellow Green instead of Goblin Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Bright Yellow Green is noticeably lighter, warmer than Goblin Green.

Can I use Goblin Green instead of Bright Yellow Green?

The same difference applies the other way round: Goblin Green is noticeably darker, 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.