Septic Green vs Light Yellow

How close are they?

ΔE 24.0, loose: different enough that you should pick on purpose. Light Yellow is noticeably lighter, more saturated, warmer than Septic Green.

Can I use Light Yellow instead of Septic Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Light Yellow is noticeably lighter, more saturated, warmer than Septic Green.

Can I use Septic Green instead of Light Yellow?

The same difference applies the other way round: Septic 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.