Bright Yellow Green vs Sick Green

How close are they?

ΔE 24.6, loose: different enough that you should pick on purpose. Sick Green is noticeably darker, more muted, cooler than Bright Yellow Green.

Can I use Sick Green instead of Bright Yellow Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Sick Green is noticeably darker, more muted, cooler than Bright Yellow Green.

Can I use Bright Yellow Green instead of Sick Green?

The same difference applies the other way round: Bright Yellow Green is noticeably lighter, more saturated, 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.